Will ePDQ work with any "off the shelf" Internet shop packages?

ePDQ should work with most Internet shopping baskets. The storefront must at least be capable of sending order details (the minimum being total value plus your ePDQ store data) using the HTTP "POST" protocol from a known fixed URL to the server. The storefront must also be capable of handling data returned from ePDQ via the HTTP "POST" protocol. Your web developer should be able to advise.