WooCommerce Purchase Order Gateway

Receive purchase orders via your WooCommerce-powered online store.
The purchase order gateway extension for WooCommerce adds a field to the checkout screen where your customer enters in their purchase order number (provided by you directly to the customer in a manual agreement).

Demo Link : https://woocommerce.com/products/woocommerce-gateway-purchase-order/

Summary from Demo
The most customizable eCommerce platform for building your online business. Get started today for free.
The Purchase Order Gateway extension for WooCommerce adds a field to the checkout screen where your customer enters in their purchase order number (provided by you directly to the customer in a manual agreement).
The Purchase Order Gateway requires no configuration to get up and running.
To tweak the instructions shown to the customer visit the WooCommerce > Settings > Payments > Purchase Order screen.
WooCommerce - the most customizable eCommerce platform for building your online business.