This plugin allows you to pay with Purchase Orders.

A purchase order (PO) is a document created by a buyer. PO displays the order details, such as quantity and types of products that buyer wants to purchase, and also contains payment terms and delivery details.

Features:

• Additional fee. Use percentage: determines whether to apply a percentage additional fee to the order total.
• Shippable product required: indicates whether shippable products are required in order to display this payment method during checkout

nopCommerce site: https://www.nopcommerce.com

Listing on nopCommerce "extensions and themes" catalog: https://www.nopcommerce.com/purchase-order