plugin for Touchnet Informations System uPay interface

I was not able to find a plugin for Touchnet so I am in the process of developing one. The uPay interface requires that I submit a web form to Touchnet (which I am simulating) with various parameters in the body of the message (sample parameters are: UPAY_SITE_ID=24&AMT=155.32 and billing city, state and zip code) by redirecting to a Touchnet website. The Touchnet web process interacts with the customer to get the credit card information then calls a fixed web page address that was specified when the merchant account was created. In this call, query string and form variables are provided that provide information about the customer and outcome of the credit card processing. A second fixed web page address is used by the Touchnet system to produce the customer's receipt.

I need pointers as to how to tell MarketPlace to allow me to redirect to the Touchnet website and how to define the two call back web addresses.

Any suggestions will be greatly appreciated.

Thanks,
Mike Cagle