AbleCommerce Shopping Cart Software

I take it the Trade Account user builds their order, enters their PO number, the order is logged to their AC account along with the PO number, and the order is dispatched, with the balance due after 30 days..

Is this synopsis correct?

Is there a way of adding this Purchase Order number to the invoice for the client??

You can access PO number via payment.ReferenceNumber. In order confirmation Email and view order page you can see that number available for customer.