diff --git a/.circleci/config.yml b/.circleci/config.yml
index f2218d74418..273f17fc5e8 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -218,6 +218,14 @@ jobs:
             cd /tmp/my_app
             bundle list | grep 'solidus_paypal_commerce_platform (1.'
 
+      - install_solidus: { flags: "--sample=false --frontend=starter --payment-method=braintree" }
+      - test_page: { expected_text: "The only eCommerce platform you’ll ever need." }
+      - run:
+          name: Ensure the correct Braintree is installed for SSF
+          command: |
+            cd /tmp/my_app
+            bundle list | grep 'solidus_braintree (3.'
+
       - install_solidus: { flags: "--sample=false --frontend=none --authentication=none" }
       - test_page: { expected_text: "<title>Ruby on Rails" }
 
diff --git a/api/app/controllers/spree/api/shipments_controller.rb b/api/app/controllers/spree/api/shipments_controller.rb
index 84985e09b23..b21a1a12008 100644
--- a/api/app/controllers/spree/api/shipments_controller.rb
+++ b/api/app/controllers/spree/api/shipments_controller.rb
@@ -127,7 +127,8 @@ def transfer_to_shipment
           current_shipment: @original_shipment,
           desired_shipment: @desired_shipment,
           variant: @variant,
-          quantity: @quantity
+          quantity: @quantity,
+          track_inventory: Spree::Config.track_inventory_levels
         )
 
         if fulfilment_changer.run!
diff --git a/backend/spec/features/admin/orders/customer_returns_spec.rb b/backend/spec/features/admin/orders/customer_returns_spec.rb
index b654d0e94b3..7ea7e6928d7 100644
--- a/backend/spec/features/admin/orders/customer_returns_spec.rb
+++ b/backend/spec/features/admin/orders/customer_returns_spec.rb
@@ -24,7 +24,7 @@ def order_state_label
     let(:order) { create :shipped_order, line_items_count: 2 }
 
     context 'when creating a return with state "Received"' do
-      it 'marks the order as returned', :js do
+      it 'marks the order as returned', :js, :flaky do
         create_customer_return('receive')
 
         expect(page).to have_content 'Customer Return has been successfully created'
@@ -41,7 +41,7 @@ def order_state_label
       expect(page).to have_button("Create", disabled: true)
     end
 
-    context 'when creating a return with state "In Transit" and then marking it as "Received"' do
+    context 'when creating a return with state "In Transit" and then marking it as "Received"', :flaky do
       it 'marks the order as returned', :js do
         create_customer_return('in_transit')
         expect(page).to have_content 'Customer Return has been successfully created'
diff --git a/backend/spec/features/admin/orders/order_details_spec.rb b/backend/spec/features/admin/orders/order_details_spec.rb
index f1688a28529..5abc6b66fcb 100644
--- a/backend/spec/features/admin/orders/order_details_spec.rb
+++ b/backend/spec/features/admin/orders/order_details_spec.rb
@@ -583,6 +583,29 @@
             expect(page).to have_content("2 x Backordered")
           end
         end
+
+        context 'when inventory levels are not being tracked' do
+          before do
+            stub_spree_preferences(track_inventory_levels: false)
+            # Simulate the default state of stock when track inventory is false.
+            shipment1.inventory_units.update_all(state: :on_hand)
+            product.master.stock_items.last.update_column(:backorderable, true)
+            product.master.stock_items.last.update_column(:count_on_hand, 0)
+          end
+
+          # Regression for https://github.com/solidusio/solidus/issues/2817
+          it "allows to ship an order's items" do
+            visit spree.edit_admin_order_path(order)
+
+            within('tr', text: line_item.sku) { click_icon 'arrows-h' }
+            complete_split_to(stock_location2)
+
+            expect(page).to have_css('.shipment', count: 2)
+            expect(order.shipments.count).to eq(2)
+            expect(page).not_to have_content('Backordered')
+            expect(page).to have_content('On hand', count: 2)
+          end
+        end
       end
 
       describe 'line item sort order' do
diff --git a/core/app/models/spree/fulfilment_changer.rb b/core/app/models/spree/fulfilment_changer.rb
index 12c5efe131b..377299f460e 100644
--- a/core/app/models/spree/fulfilment_changer.rb
+++ b/core/app/models/spree/fulfilment_changer.rb
@@ -17,18 +17,27 @@ module Spree
   # @attr [Integer] quantity How many units we want to move
   #
   class FulfilmentChanger
+    TRACK_INVENTORY_NOT_PROVIDED = Object.new.freeze
     include ActiveModel::Validations
 
     attr_accessor :current_shipment, :desired_shipment
-    attr_reader :variant, :quantity, :current_stock_location, :desired_stock_location
+    attr_reader :variant, :quantity, :current_stock_location, :desired_stock_location, :track_inventory
 
-    def initialize(current_shipment:, desired_shipment:, variant:, quantity:)
+    def initialize(current_shipment:, desired_shipment:, variant:, quantity:, track_inventory: TRACK_INVENTORY_NOT_PROVIDED)
       @current_shipment = current_shipment
       @desired_shipment = desired_shipment
       @current_stock_location = current_shipment.stock_location
       @desired_stock_location = desired_shipment.stock_location
       @variant = variant
       @quantity = quantity
+      @track_inventory = if track_inventory == TRACK_INVENTORY_NOT_PROVIDED
+        Spree::Deprecation.warn(
+          "Not passing `track_inventory` to `Spree::FulfilmentChanger` is deprecated." \
+        )
+        true
+      else
+        track_inventory
+      end
     end
 
     validates :quantity, numericality: { greater_than: 0 }
@@ -45,6 +54,45 @@ def run!
       return false if invalid?
       desired_shipment.save! if desired_shipment.new_record?
 
+      if track_inventory
+        run_tracking_inventory
+      else
+        run_without_tracking_inventory
+      end
+
+      # We modified the inventory units at the database level for speed reasons.
+      # The downside of that is that we need to reload the associations.
+      current_shipment.inventory_units.reload
+      desired_shipment.inventory_units.reload
+
+      # If the current shipment now has no inventory units left, we won't need it any longer.
+      if current_shipment.inventory_units.length.zero?
+        current_shipment.destroy!
+      else
+        # The current shipment has changed, so we need to make sure that shipping rates
+        # have the correct amount.
+        current_shipment.refresh_rates
+      end
+
+      # The desired shipment has also change, so we need to make sure shipping rates
+      # are up-to-date, too.
+      desired_shipment.refresh_rates
+
+      # In order to reflect the changes in the order totals
+      desired_shipment.order.reload
+      desired_shipment.order.recalculate
+
+      true
+    end
+
+    private
+
+    # When moving things from one stock location to another, we need to restock items
+    # from the current location and unstock them at the desired location.
+    # Also, when we want to track inventory changes, we need to make sure that we have
+    # enough stock at the desired location to fulfil the order. Based on how many items
+    # we can take from the desired location, we could end up with some items being backordered.
+    def run_tracking_inventory
       # Retrieve how many on hand items we can take from desired stock location
       available_quantity = [desired_shipment.stock_location.count_on_hand(variant), default_on_hand_quantity].max
 
@@ -79,34 +127,21 @@ def run!
           limit(quantity - new_on_hand_quantity).
           update_all(shipment_id: desired_shipment.id, state: :backordered)
       end
+    end
 
-      # We modified the inventory units at the database level for speed reasons.
-      # The downside of that is that we need to reload the associations.
-      current_shipment.inventory_units.reload
-      desired_shipment.inventory_units.reload
-
-      # If the current shipment now has no inventory units left, we won't need it any longer.
-      if current_shipment.inventory_units.length.zero?
-        current_shipment.destroy!
-      else
-        # The current shipment has changed, so we need to make sure that shipping rates
-        # have the correct amount.
-        current_shipment.refresh_rates
+    # When we don't track inventory, we can just move the inventory units from one shipment
+    # to the other.
+    def run_without_tracking_inventory
+      ActiveRecord::Base.transaction do
+        current_shipment.
+          inventory_units.
+          where(variant: variant).
+          order(state: :asc).
+          limit(quantity).
+          update_all(shipment_id: desired_shipment.id)
       end
-
-      # The desired shipment has also change, so we need to make sure shipping rates
-      # are up-to-date, too.
-      desired_shipment.refresh_rates
-
-      # In order to reflect the changes in the order totals
-      desired_shipment.order.reload
-      desired_shipment.order.recalculate
-
-      true
     end
 
-    private
-
     # We don't need to handle stock counts for incomplete orders. Also, if
     # the new shipment and the desired shipment will ship from the same stock location,
     # unstocking and restocking will not be necessary.
diff --git a/core/app/models/spree/product/scopes.rb b/core/app/models/spree/product/scopes.rb
index 6daa94c35b3..bcaeac5824a 100644
--- a/core/app/models/spree/product/scopes.rb
+++ b/core/app/models/spree/product/scopes.rb
@@ -158,7 +158,7 @@ def self.property_conditions(property)
           # order: 'COALESCE(cnt, 0) DESC'
           add_search_scope :descend_by_popularity do
             joins(:master).
-              order(%{
+              order(Arel.sql(%{
            COALESCE((
              SELECT
                COUNT(#{Spree::LineItem.quoted_table_name}.id)
@@ -171,7 +171,7 @@ def self.property_conditions(property)
              WHERE
                popular_variants.product_id = #{Spree::Product.quoted_table_name}.id
            ), 0) DESC
-        })
+        }))
           end
 
           add_search_scope :not_deleted do
diff --git a/core/app/models/spree/promotion.rb b/core/app/models/spree/promotion.rb
index 6b4794b7a88..999e79b0db6 100644
--- a/core/app/models/spree/promotion.rb
+++ b/core/app/models/spree/promotion.rb
@@ -2,7 +2,8 @@
 
 module Spree
   class Promotion < Spree::Base
-    MATCH_POLICIES = %w(all any)
+
+    autoload(:MATCH_POLICIES, "spree/promotion/match_policies")
 
     UNACTIVATABLE_ORDER_STATES = ["complete", "awaiting_return", "returned"]
 
diff --git a/core/app/models/spree/taxon.rb b/core/app/models/spree/taxon.rb
index 48ddaf7648e..13a9c7e1f70 100644
--- a/core/app/models/spree/taxon.rb
+++ b/core/app/models/spree/taxon.rb
@@ -18,9 +18,11 @@ class Taxon < Spree::Base
     after_update :update_child_permalinks, if: :saved_change_to_permalink?
 
     validates :name, presence: true
+    validates :name, uniqueness: { scope: :parent_id, message: :must_be_unique_under_same_parent }, if: -> { Spree::Config.extra_taxon_validations}
     validates :meta_keywords, length: { maximum: 255 }
     validates :meta_description, length: { maximum: 255 }
     validates :meta_title, length: { maximum: 255 }
+    validates :taxonomy_id, uniqueness: { message: :can_have_only_one_root }, if: -> { Spree::Config.extra_taxon_validations && root? }
 
     after_save :touch_ancestors_and_taxonomy
     after_touch :touch_ancestors_and_taxonomy
diff --git a/core/app/models/spree/taxonomy.rb b/core/app/models/spree/taxonomy.rb
index bbaeddcba25..37f6f4acfcc 100644
--- a/core/app/models/spree/taxonomy.rb
+++ b/core/app/models/spree/taxonomy.rb
@@ -5,6 +5,7 @@ class Taxonomy < Spree::Base
     acts_as_list
 
     validates :name, presence: true
+    validates :name, uniqueness: true, if: -> { Spree::Config.extra_taxonomy_validations }
 
     has_many :taxons, inverse_of: :taxonomy
     has_one :root, -> { where parent_id: nil }, class_name: "Spree::Taxon", dependent: :destroy
diff --git a/core/config/locales/en.yml b/core/config/locales/en.yml
index 1d54e09b5e4..a9d4d95b6e6 100644
--- a/core/config/locales/en.yml
+++ b/core/config/locales/en.yml
@@ -498,6 +498,12 @@ en:
           attributes:
             base:
               cannot_destroy_default_store: Cannot destroy the default Store.
+        spree/taxon:
+          attributes:
+            name:
+              must_be_unique_under_same_parent: must be unique under the same parent Taxon
+            taxonomy_id:
+              can_have_only_one_root: can only have one root Taxon
         spree/user_address:
           attributes:
             user_id:
diff --git a/core/lib/generators/solidus/install/app_templates/payment_method/braintree.rb b/core/lib/generators/solidus/install/app_templates/payment_method/braintree.rb
new file mode 100644
index 00000000000..a3bebb93132
--- /dev/null
+++ b/core/lib/generators/solidus/install/app_templates/payment_method/braintree.rb
@@ -0,0 +1,5 @@
+unless Bundler.locked_gems.dependencies['solidus_braintree']
+  bundle_command 'add solidus_braintree --version "~> 3.0"'
+end
+
+generate 'solidus_braintree:install'
diff --git a/core/lib/generators/solidus/install/install_generator.rb b/core/lib/generators/solidus/install/install_generator.rb
index 831b53bdca6..fb82b5edb8f 100644
--- a/core/lib/generators/solidus/install/install_generator.rb
+++ b/core/lib/generators/solidus/install/install_generator.rb
@@ -29,10 +29,31 @@ class InstallGenerator < Rails::Generators::AppBase
       none
     ]
 
-    PAYMENT_METHODS = %w[
-      paypal
-      bolt
-      none
+    PAYMENT_METHODS = [
+      {
+        name: 'paypal',
+        frontends: %w[none classic starter],
+        description: 'Install `solidus_paypal_commerce_platform`',
+        default: true,
+      },
+      {
+        name: 'bolt',
+        frontends: %w[classic],
+        description: 'Install `solidus_bolt`',
+        default: false,
+      },
+      {
+        name: 'braintree',
+        frontends: %w[none starter],
+        description: 'Install `solidus_braintree`',
+        default: false,
+      },
+      {
+        name: 'none',
+        frontends: %w[none classic starter],
+        description: 'Skip installing a payment method',
+        default: false,
+      },
     ]
 
     class_option :migrate, type: :boolean, default: true, banner: 'Run Solidus migrations'
@@ -48,7 +69,7 @@ class InstallGenerator < Rails::Generators::AppBase
 
     class_option :frontend, type: :string, enum: FRONTENDS + LEGACY_FRONTENDS, default: nil, desc: "Indicates which frontend to install."
     class_option :authentication, type: :string, enum: AUTHENTICATIONS, default: nil, desc: "Indicates which authentication system to install."
-    class_option :payment_method, type: :string, enum: PAYMENT_METHODS, default: nil, desc: "Indicates which payment method to install."
+    class_option :payment_method, type: :string, enum: PAYMENT_METHODS.map { |payment_method| payment_method[:name] }, default: nil, desc: "Indicates which payment method to install."
 
     # DEPRECATED
     class_option :with_authentication, type: :boolean, hide: true, default: nil
@@ -323,29 +344,26 @@ def detect_payment_method_to_install
       return 'paypal' if Bundler.locked_gems.dependencies['solidus_paypal_commerce_platform']
       return 'bolt' if Bundler.locked_gems.dependencies['solidus_bolt']
 
-      descriptions = {
-        paypal: "- [#{set_color 'paypal', :bold}] Install `solidus_paypal_commerce_platform` (#{set_color :default, :bold}).",
-        bolt: "- [#{set_color 'bolt', :bold}] Install `solidus_bolt`.",
-        none: "- [#{set_color 'none', :bold}] Skip installing a payment method.",
-      }
-
-      payment_methods = PAYMENT_METHODS
-
-      if @selected_frontend != 'classic'
-        payment_methods -= ['bolt']
-        descriptions.delete(:bolt)
+      selected_frontend_payment_methods = PAYMENT_METHODS.select do |payment_method|
+        payment_method[:frontends].include?(@selected_frontend)
       end
 
       selected = options[:payment_method] || (options[:auto_accept] && 'paypal') ||
         ask_with_description(
           default: 'paypal',
-          limited_to: payment_methods,
+          limited_to: selected_frontend_payment_methods.map { |payment_method| payment_method[:name] },
           desc: <<~TEXT
             Which payment method would you like to use?
 
-            #{descriptions.values.join("\n")}
+            #{selected_frontend_payment_methods.map { |payment_method| formatted_payment_method_description(payment_method) }.join("\n")}
           TEXT
         )
     end
+
+    def formatted_payment_method_description(payment_method)
+      default_label = " (#{set_color :default, :bold})" if payment_method[:default]
+
+      "- [#{set_color payment_method[:name], :bold}] #{payment_method[:description]}#{default_label}."
+    end
   end
 end
diff --git a/core/lib/generators/solidus/update/update_generator.rb b/core/lib/generators/solidus/update/update_generator.rb
index b32f1c00b14..8f448fffa7c 100644
--- a/core/lib/generators/solidus/update/update_generator.rb
+++ b/core/lib/generators/solidus/update/update_generator.rb
@@ -8,7 +8,7 @@ module Solidus
   class UpdateGenerator < ::Rails::Generators::Base
     FROM = Spree.previous_solidus_minor_version
 
-    desc 'Generates a new initializer to preview the new defaults for current Solidus version'
+    desc 'Generates a new initializer to preview the new defaults for current Solidus version and copy new migrations'
 
     source_root File.expand_path('templates', __dir__)
 
@@ -37,10 +37,15 @@ class UpdateGenerator < ::Rails::Generators::Base
                  default: 'config/initializers/',
                  hide: true
 
+    class_option :install_migrations,
+                 type: :boolean,
+                 default: true,
+                 hide: true
+
     def create_new_defaults_initializer
       previous_version_prompt = options[:previous_version_prompt]
       return if previous_version_prompt && !yes?(<<~MSG, :red)
-        The update process is only supported if you are coming from version #{FROM}. If this is not the case, please, skip it and update your application to use Solidus #{FROM} before retrying.
+        The default preferences update process is only supported if you are coming from version #{FROM}. If this is not the case, please, skip it and update your application to use Solidus #{FROM} before retrying.
         If you are confident you want to upgrade from a previous version, you must rerun the generator with the "--from={OLD_VERSION}" argument.
         Are you sure you want to continue? (y/N)
       MSG
@@ -57,6 +62,13 @@ def create_new_defaults_initializer
                File.join(options[:initializer_directory], "#{options[:initializer_basename]}.rb")
     end
 
+    def install_migrations
+      return unless options[:install_migrations]
+
+      say_status :copying, "migrations"
+      rake 'spree:install:migrations'
+    end
+
     def print_message
       say <<~MSG
 
diff --git a/core/lib/spree/app_configuration.rb b/core/lib/spree/app_configuration.rb
index 169727b21fd..05649689168 100644
--- a/core/lib/spree/app_configuration.rb
+++ b/core/lib/spree/app_configuration.rb
@@ -65,6 +65,22 @@ class AppConfiguration < Preferences::Configuration
     #   @return [Boolean] When false, admins cannot create promotions with an "any" match policy (default: +false+)
     #                     Create individual, separate promotions for each of your rules instead.
     preference :allow_promotions_any_match_policy, :boolean, default: false
+    def allow_promotions_any_match_policy=(value)
+      if value == true
+        Spree::Deprecation.warn <<~MSG
+          Solidus 4.0 will remove support for combining promotion rules with the "any" match policy.
+
+          Instead, it's suggested to create individual, separate promotions for each of your current
+          rules combined with the "any" policy. To automate this task, you can use the provided
+          task:
+
+                  bin/rake solidus:split_promotions_with_any_match_policy
+        MSG
+      end
+
+      preferences[:allow_promotions_any_match_policy] = value
+    end
+
 
     # @!attribute [rw] guest_token_cookie_options
     #   @return [Hash] Add additional guest_token cookie options here (ie. domain or path)
@@ -155,6 +171,38 @@ class AppConfiguration < Preferences::Configuration
     #   @return [Regexp] Regex to be used in email validations, for example in Spree::EmailValidator
     preference :default_email_regexp, :regexp, default: URI::MailTo::EMAIL_REGEXP
 
+    # @!attribute [rw] extra_taxon_validations
+    #   Use extra validations on Taxons in 3.4.0, but default to false so stores can
+    #   upgrade and then fix any now invalid Taxons before enabling.
+    #   @return [Boolean]
+    versioned_preference :extra_taxon_validations, :boolean, initial_value: false, boundaries: { "3.4.0.dev" => true }
+    def extra_taxon_validations=(value)
+      Spree::Deprecation.warn <<~MSG
+        Solidus will remove `Spree::Config.extra_taxon_validations` preference
+        in the next major release and will always use the extra Taxon validations.
+        Before upgrading to the next major, please fix any now invalid Taxons
+        and then remove the preference definition in `config/initializers/spree.rb`.
+      MSG
+
+      preferences[:extra_taxon_validations] = value
+    end
+
+    # @!attribute [rw] extra_taxonomy_validations
+    #   Use extra validations on Taxonomies in 3.4.0, but default to false so stores can
+    #   upgrade and then fix any now invalid Taxonomies before enabling.
+    #   @return [Boolean]
+    versioned_preference :extra_taxonomy_validations, :boolean, initial_value: false, boundaries: { "3.4.0.dev" => true }
+    def extra_taxonomy_validations=(value)
+      Spree::Deprecation.warn <<~MSG
+        Solidus will remove `Spree::Config.extra_taxonomy_validations` preference
+        in the next major release and will always use the extra Taxonomy validations.
+        Before upgrading to the next major, please fix any now invalid Taxons
+        and then remove the preference definition in `config/initializers/spree.rb`.
+      MSG
+
+      preferences[:extra_taxonomy_validations] = value
+    end
+
     # @!attribute [rw] generate_api_key_for_all_roles
     #   @return [Boolean] Allow generating api key automatically for user
     #   at role_user creation for all roles. (default: +false+)
diff --git a/core/lib/spree/core/product_duplicator.rb b/core/lib/spree/core/product_duplicator.rb
index f2ba0d9383b..882592a3596 100644
--- a/core/lib/spree/core/product_duplicator.rb
+++ b/core/lib/spree/core/product_duplicator.rb
@@ -45,7 +45,7 @@ def duplicate_master
         new_master.sku = "COPY OF #{master.sku}"
         new_master.deleted_at = nil
         new_master.images = master.images.map { |image| duplicate_image image } if @include_images
-        new_master.price = master.price
+        new_master.prices = master.prices.map(&:dup)
       end
     end
 
diff --git a/core/lib/spree/promotion/match_policies.rb b/core/lib/spree/promotion/match_policies.rb
new file mode 100644
index 00000000000..712dbd029c9
--- /dev/null
+++ b/core/lib/spree/promotion/match_policies.rb
@@ -0,0 +1,2 @@
+Spree::Promotion::MATCH_POLICIES = %w(all any)
+Spree::Deprecation.warn('Spree::Promotion::MATCH_POLICIES is deprecated')
diff --git a/core/lib/spree/testing_support/factories/taxon_factory.rb b/core/lib/spree/testing_support/factories/taxon_factory.rb
index 8f1527a5b23..1d2a94752af 100644
--- a/core/lib/spree/testing_support/factories/taxon_factory.rb
+++ b/core/lib/spree/testing_support/factories/taxon_factory.rb
@@ -10,11 +10,13 @@
 FactoryBot.define do
   factory :taxon, class: 'Spree::Taxon' do
     name { 'Ruby on Rails' }
-    taxonomy
-    parent_id { nil }
+    taxonomy_id { (parent&.taxonomy || create(:taxonomy)).id }
+    parent_id { parent&.id || taxonomy.root.id }
 
     trait :with_icon do
-      icon { Spree::Core::Engine.root.join('lib', 'spree', 'testing_support', 'fixtures', 'blank.jpg').open }
+      after(:create) do |taxon|
+        taxon.update(icon: Spree::Core::Engine.root.join('lib', 'spree', 'testing_support', 'fixtures', 'blank.jpg').open)
+      end
     end
   end
 end
diff --git a/core/lib/spree/testing_support/factories/taxonomy_factory.rb b/core/lib/spree/testing_support/factories/taxonomy_factory.rb
index 029f2c86669..da69e768f71 100644
--- a/core/lib/spree/testing_support/factories/taxonomy_factory.rb
+++ b/core/lib/spree/testing_support/factories/taxonomy_factory.rb
@@ -7,6 +7,8 @@
 
 FactoryBot.define do
   factory :taxonomy, class: 'Spree::Taxonomy' do
-    name { 'Brand' }
+    sequence :name do |seq|
+      "Brand #{seq}"
+    end
   end
 end
diff --git a/core/spec/generators/solidus/install/install_generator_spec.rb b/core/spec/generators/solidus/install/install_generator_spec.rb
index 88f4b7ce26f..fe110bdc526 100644
--- a/core/spec/generators/solidus/install/install_generator_spec.rb
+++ b/core/spec/generators/solidus/install/install_generator_spec.rb
@@ -110,10 +110,11 @@
         expect(questions.first[:default]).to eq('paypal')
         expect(strip_ansi questions.first[:desc]).to include('[paypal]')
         expect(strip_ansi questions.first[:desc]).to include('[bolt]')
+        expect(strip_ansi questions.first[:desc]).to_not include('[braintree]')
         expect(strip_ansi questions.first[:desc]).to include('[none]')
       end
 
-      it 'presents different options for the "classic"' do
+      it 'presents different options for the "starter"' do
         questions = []
         generator = described_class.new([], ['--frontend=starter', '--authentication=devise'])
         allow(generator).to receive(:ask_with_description) { |**args| questions << args }
@@ -121,10 +122,11 @@
         generator.prepare_options
 
         expect(questions.size).to eq(1)
-        expect(questions.first[:limited_to]).to eq(['paypal', 'none'])
+        expect(questions.first[:limited_to]).to eq(['paypal', 'braintree', 'none'])
         expect(questions.first[:default]).to eq('paypal')
         expect(strip_ansi questions.first[:desc]).to include('[paypal]')
         expect(strip_ansi questions.first[:desc]).not_to include('[bolt]')
+        expect(strip_ansi questions.first[:desc]).to include('[braintree]')
         expect(strip_ansi questions.first[:desc]).to include('[none]')
       end
     end
diff --git a/core/spec/lib/generators/solidus/update/update_generator_spec.rb b/core/spec/lib/generators/solidus/update/update_generator_spec.rb
index c7af360bedb..d2d05146e4d 100644
--- a/core/spec/lib/generators/solidus/update/update_generator_spec.rb
+++ b/core/spec/lib/generators/solidus/update/update_generator_spec.rb
@@ -12,6 +12,7 @@
       Rails::Generators.invoke('solidus:update', [
         "--initializer_directory=#{Rails.root.join('tmp')}",
         "--previous_version_prompt=false",
+        "--install_migrations=false",
         "--from=#{from}",
         "--to=#{to}",
         "--quiet"
diff --git a/core/spec/lib/spree/app_configuration_spec.rb b/core/spec/lib/spree/app_configuration_spec.rb
index db5fe174f4f..6d2cf5efef7 100644
--- a/core/spec/lib/spree/app_configuration_spec.rb
+++ b/core/spec/lib/spree/app_configuration_spec.rb
@@ -177,4 +177,36 @@ class DummyClass; end;
       expect(prefs.mails_from).to eq('solidus@example.com')
     end
   end
+
+  context 'extra_taxon_validations=' do
+    it 'is deprecated' do
+      expect(Spree::Deprecation).to receive(:warn).with(/Solidus will remove `Spree::Config.extra_taxon_validations`/)
+
+      prefs.extra_taxon_validations=(false)
+    end
+
+    it "still sets the value so that consumers aren't broken" do
+      Spree::Deprecation.silence do
+        prefs.extra_taxon_validations=(false)
+      end
+
+      expect(prefs.extra_taxon_validations).to eq(false)
+    end
+  end
+
+  context 'extra_taxonomy_validations=' do
+    it 'is deprecated' do
+      expect(Spree::Deprecation).to receive(:warn).with(/Solidus will remove `Spree::Config.extra_taxonomy_validations`/)
+
+      prefs.extra_taxonomy_validations=(false)
+    end
+
+    it "still sets the value so that consumers aren't broken" do
+      Spree::Deprecation.silence do
+        prefs.extra_taxonomy_validations=(false)
+      end
+
+      expect(prefs.extra_taxonomy_validations).to eq(false)
+    end
+  end
 end
diff --git a/core/spec/lib/spree/core/testing_support/factories/taxon_factory_spec.rb b/core/spec/lib/spree/core/testing_support/factories/taxon_factory_spec.rb
index b24fdadf92e..6ee5008495b 100644
--- a/core/spec/lib/spree/core/testing_support/factories/taxon_factory_spec.rb
+++ b/core/spec/lib/spree/core/testing_support/factories/taxon_factory_spec.rb
@@ -9,5 +9,46 @@
     let(:factory) { :taxon }
 
     it_behaves_like 'a working factory'
+
+    context "when no taxonomy is given" do
+      subject { create(:taxon) }
+
+      before do
+        # ensure that the subject is not accidentally
+        # getting valid parent and taxonomy ids
+        create(:taxon)
+      end
+
+      it "sets its taxonomy to created one and its parent to the taxonomy root" do
+        expect(subject).to be_valid
+        expect(subject.taxonomy).to be_present
+        expect(subject.parent).to eq(subject.taxonomy.root)
+        expect(subject).to_not eq(subject.taxonomy.root)
+      end
+    end
+
+    context "when a taxonomy is given" do
+      subject { create(:taxon, taxonomy: taxonomy) }
+
+      let!(:taxonomy) { create(:taxonomy) }
+
+      it "sets the taxonomy to the given one and its parent to the taxonomy root" do
+        expect(subject).to be_valid
+        expect(subject.taxonomy).to eq(taxonomy)
+        expect(subject.parent).to eq(taxonomy.root)
+      end
+    end
+
+    context "when a parent is given" do
+      subject { create(:taxon, parent: parent_taxon) }
+
+      let!(:parent_taxon) { create(:taxon) }
+
+      it "sets the parent to the given one and its taxonomy to the parent's taxonomy" do
+        expect(subject).to be_valid
+        expect(subject.parent).to eq(parent_taxon)
+        expect(subject.taxonomy).to eq(parent_taxon.taxonomy)
+      end
+    end
   end
 end
diff --git a/core/spec/models/spree/fulfilment_changer_spec.rb b/core/spec/models/spree/fulfilment_changer_spec.rb
index 897d82835cd..1ff25e76453 100644
--- a/core/spec/models/spree/fulfilment_changer_spec.rb
+++ b/core/spec/models/spree/fulfilment_changer_spec.rb
@@ -4,6 +4,7 @@
 
 RSpec.describe Spree::FulfilmentChanger do
   let(:variant) { create(:variant) }
+  let(:track_inventory) { true }
 
   let(:order) do
     create(
@@ -20,13 +21,16 @@
   let(:current_shipment) { order.shipments.first }
   let(:desired_shipment) { order.shipments.create!(stock_location: desired_stock_location) }
   let(:desired_stock_location) { current_shipment.stock_location }
+  let(:current_shipment_inventory_unit_count) { 1 }
+  let(:quantity) { current_shipment_inventory_unit_count }
 
   let(:shipment_splitter) do
     described_class.new(
       current_shipment: current_shipment,
       desired_shipment: desired_shipment,
       variant: variant,
-      quantity: quantity
+      quantity: quantity,
+      track_inventory: track_inventory
     )
   end
 
@@ -38,9 +42,6 @@
   end
 
   context "when the current shipment stock location is the same of the target shipment" do
-    let(:current_shipment_inventory_unit_count) { 1 }
-    let(:quantity) { current_shipment_inventory_unit_count }
-
     context "when the stock location is empty" do
       before do
         variant.stock_items.first.update_column(:count_on_hand, 0)
@@ -70,6 +71,122 @@
     end
   end
 
+  context "when track_inventory is not passed" do
+    let(:shipment_splitter) do
+      described_class.new(
+        current_shipment: current_shipment,
+        desired_shipment: desired_shipment,
+        variant: variant,
+        quantity: quantity
+      )
+    end
+
+    it "defaults to true with a deprecation warning" do
+      expect(Spree::Deprecation).to receive(:warn).with(/Not passing `track_inventory` to `Spree::FulfilmentChanger` is deprecated./)
+      expect(shipment_splitter.track_inventory).to be true
+    end
+  end
+
+  context "when tracking inventory is not set (same as false)" do
+    let(:current_shipment_inventory_unit_count) { 2 }
+    let(:quantity) { 1 }
+    let(:track_inventory) { nil }
+
+    it "adds the desired inventory units to the desired shipment" do
+      expect { subject }.to change { desired_shipment.inventory_units.length }.by(quantity)
+    end
+
+    it "removes the desired inventory units from the current shipment" do
+      expect { subject }.to change { current_shipment.inventory_units.length }.by(-quantity)
+    end
+
+    it "recalculates shipping costs for the current shipment" do
+      expect(current_shipment).to receive(:refresh_rates)
+      subject
+    end
+
+    it 'updates order totals' do
+      original_total = order.total
+      original_shipment_total = order.shipment_total
+
+      expect { subject }.
+        to change { order.total }.from(original_total).to(original_total + original_shipment_total).
+        and change { order.shipment_total }.by(original_shipment_total)
+    end
+
+    context "when transferring to another stock location" do
+      let(:desired_stock_location) { create(:stock_location) }
+      let!(:stock_item) do
+        variant.stock_items.find_or_create_by!(
+          stock_location: desired_stock_location,
+          variant: variant,
+        )
+      end
+
+      it "is marked as a successful transfer" do
+        expect(subject).to be true
+      end
+
+      it "does not stock in the current stock location" do
+        expect { subject }.not_to change { current_shipment.stock_location.count_on_hand(variant) }
+      end
+
+      it "does not unstock the desired stock location" do
+        expect { subject }.not_to change { desired_shipment.stock_location.count_on_hand(variant) }
+      end
+    end
+  end
+
+  context "when not tracking inventory" do
+    let(:current_shipment_inventory_unit_count) { 2 }
+    let(:quantity) { 1 }
+    let(:track_inventory) { false }
+
+    it "adds the desired inventory units to the desired shipment" do
+      expect { subject }.to change { desired_shipment.inventory_units.length }.by(quantity)
+    end
+
+    it "removes the desired inventory units from the current shipment" do
+      expect { subject }.to change { current_shipment.inventory_units.length }.by(-quantity)
+    end
+
+    it "recalculates shipping costs for the current shipment" do
+      expect(current_shipment).to receive(:refresh_rates)
+      subject
+    end
+
+    it 'updates order totals' do
+      original_total = order.total
+      original_shipment_total = order.shipment_total
+
+      expect { subject }.
+        to change { order.total }.from(original_total).to(original_total + original_shipment_total).
+        and change { order.shipment_total }.by(original_shipment_total)
+    end
+
+    context "when transferring to another stock location" do
+      let(:desired_stock_location) { create(:stock_location) }
+      let!(:stock_item) do
+        variant.stock_items.find_or_create_by!(
+          stock_location: desired_stock_location,
+          variant: variant,
+        )
+      end
+
+      it "is marked as a successful transfer" do
+        expect(subject).to be true
+      end
+
+      it "does not stock in the current stock location" do
+        expect { subject }.not_to change { current_shipment.stock_location.count_on_hand(variant) }
+      end
+
+      it "does not unstock the desired stock location" do
+        expect { subject }.not_to change { desired_shipment.stock_location.count_on_hand(variant) }
+      end
+    end
+  end
+
   context "when the current shipment has enough inventory units" do
     let(:current_shipment_inventory_unit_count) { 2 }
     let(:quantity) { 1 }
diff --git a/core/spec/models/spree/product/scopes_spec.rb b/core/spec/models/spree/product/scopes_spec.rb
index e5699e320ec..9be70c23383 100644
--- a/core/spec/models/spree/product/scopes_spec.rb
+++ b/core/spec/models/spree/product/scopes_spec.rb
@@ -116,6 +116,18 @@
     end
   end
 
+  context "descend_by_popularity" do
+    it "orders products by popularity" do
+      variant_1 = create(:master_variant)
+      variant_2 = create(:master_variant)
+
+      create_list(:line_item, 3, variant: variant_1)
+      create_list(:line_item, 2, variant: variant_2)
+
+      expect(Spree::Product.descend_by_popularity.map(&:id)).to eq([variant_1.product.id, variant_2.product.id, product.id])
+    end
+  end
+
   describe '.available' do
     context "a product with past available_on" do
       let!(:product) { create(:product, available_on: 1.day.ago) }
diff --git a/core/spec/models/spree/product_duplicator_spec.rb b/core/spec/models/spree/product_duplicator_spec.rb
index 7965b556b3c..e5720c0642a 100644
--- a/core/spec/models/spree/product_duplicator_spec.rb
+++ b/core/spec/models/spree/product_duplicator_spec.rb
@@ -61,6 +61,10 @@ module Spree
         expect(new_product.name).to eql "COPY OF #{product.name}"
       end
 
+      it "will set the same price" do
+        expect(new_product.reload.price).to eql product.price
+      end
+
       it "will set an unique sku" do
         expect(new_product.sku).to include "COPY OF SKU"
       end
diff --git a/core/spec/models/spree/promotion_spec.rb b/core/spec/models/spree/promotion_spec.rb
index 1dcdc483cae..ff848a65299 100644
--- a/core/spec/models/spree/promotion_spec.rb
+++ b/core/spec/models/spree/promotion_spec.rb
@@ -993,4 +993,11 @@
       expect(order.adjustment_total).to eq(-10)
     end
   end
+
+  describe "MATCH_POLICIES" do
+    it "prints a deprecation warning when used" do
+      expect(Spree::Deprecation).to receive(:warn).once.with(/Spree::Promotion::MATCH_POLICIES is deprecated/)
+      expect(Spree::Promotion::MATCH_POLICIES).to eq %w(all any)
+    end
+  end
 end
diff --git a/core/spec/models/spree/taxon_spec.rb b/core/spec/models/spree/taxon_spec.rb
index 8e92960bf04..6040ad53474 100644
--- a/core/spec/models/spree/taxon_spec.rb
+++ b/core/spec/models/spree/taxon_spec.rb
@@ -53,7 +53,8 @@
   end
 
   context "set_permalink" do
-    let(:taxon) { FactoryBot.build(:taxon, name: "Ruby on Rails") }
+    let(:taxonomy) { create(:taxonomy, name: "Ruby on Rails") }
+    let(:taxon) { taxonomy.root }
 
     it "should set permalink correctly when no parent present" do
       taxon.set_permalink
@@ -175,12 +176,36 @@
     end
   end
 
-  # Regression test for https://github.com/spree/spree/issues/2620
-  context "creating a child node using first_or_create" do
-    let(:taxonomy) { create(:taxonomy) }
+  context "validations" do
+    context "taxonomy_id validations" do
+      let(:taxonomy) { create(:taxonomy) }
+      let(:taxon) { taxonomy.taxons.create(name: 'New node') }
+
+      it "ensures that only one root can be created" do
+        expect(taxon).to be_invalid
+        expect(taxon.errors.full_messages).to match_array(["Taxonomy can only have one root Taxon"])
+      end
+
+      it "allows for multiple taxons under taxonomy" do
+        expect(taxon.update(parent_id: taxonomy.root.id)).to eq(true)
+        expect(taxonomy.taxons.many?).to eq(true)
+      end
+    end
+
+    context "name validations" do
+      let!(:taxonomy) { create(:taxonomy) }
+      let!(:taxon_level_one) { create(:taxon, name: 'Solidus', parent: taxonomy.root) }
+      let(:taxon_level_one_duplicate) { build(:taxon, name: 'Solidus', parent: taxonomy.root) }
+      let(:taxon_level_two) { create(:taxon, name: 'Solidus', parent: taxon_level_one) }
 
-    it "does not error out" do
-      taxonomy.root.children.unscoped.where(name: "Some name").first_or_create
+      it "ensures that taxons with the same parent must have unique names" do
+        expect(taxon_level_one_duplicate.save).to eq(false)
+        expect(taxon_level_one_duplicate.errors.full_messages).to match_array(["Name must be unique under the same parent Taxon"])
+      end
+
+      it "allows for multiple taxons with the same name under different parents" do
+        expect(taxon_level_two).to be_valid
+      end
     end
   end
 
diff --git a/core/spec/models/spree/taxonomy_spec.rb b/core/spec/models/spree/taxonomy_spec.rb
index 131ce22361a..4494653d0a4 100644
--- a/core/spec/models/spree/taxonomy_spec.rb
+++ b/core/spec/models/spree/taxonomy_spec.rb
@@ -3,6 +3,19 @@
 require 'rails_helper'
 
 RSpec.describe Spree::Taxonomy, type: :model do
+  context "validations" do
+    subject { build(:taxonomy, name: 'Brand') }
+
+    let!(:taxonomy) { create(:taxonomy, name: 'Brand') }
+
+    context "name validations" do
+      it "ensures Taxonomies must have unique names" do
+        expect(subject.save).to eq(false)
+        expect(subject.errors.full_messages).to match_array(["Name has already been taken"])
+      end
+    end
+  end
+
   context "#destroy" do
     subject(:association_options) do
       described_class.reflect_on_association(:root).options