Check/money order plugin allows you to send a purchase order to your customers.
Purchase orders are often used by government agencies or large businesses. Rather than paying directly through your site, shoppers will request that you send them a Purchase order (PO), and they will send the payment back. Most of the order processing is handled outside of the software.
Installation instructions:
Please find more information about how to install plugins here.
Source code:
The plugin is provided absolutely for free and includes source code.