Wow, there is a Fee for the interface:
In addition to the annual fee for your ProPay Account, additional fees apply for the XML Interface. The fees for the XML Authorization Interface is $395.00 for the first year and $195.00 each year thereafter.
ProPay is normally used to process payments where you enter the customer`s info yourself. While it can be integated for ecommerce use, it would require lots of additional coding. ProPay does offer an XML Interface that allows you to link your web site or shopping cart to your ProPay Account using XML code however, they don`t offer a "buy button or shopping cart integration.