Sometimes a product might be sold manually to an offline customer, in that case, the seller provides him an order number. That order number can be entered at the time of making payment on the web store. This will relieve the customer for making another payment on the website, as he has already paid to the seller by offline modes. The configuration of Purchase Order Gateway extension is very easy.
This is the EXACT same plugin as distributed by the developer on WooCommerce. The developers’ links are provided above.