Payment module implementation to replace PayPal

We have a php based reservation system running in Joomla with separate payment module implementations. At the moment we have payment modules for PayPal and Authorize.net. We need implementation for a new payment module for another payment provider. New module should be able to fill the the required post forms correctly as well as validate the incoming request according to the interface specification. Information coming from and fed into our reservation system should not require any changes. There is also no need for any UI implementation since payment gateway will take care of that. There is english language interface specification + reference implementations available for osCommerce and Zencart for constructing and handling the required messages. You can also get our current implementations for PayPal and [login to view URL] as a reference for message handling in our system. Interface specification is attached, you can PM us for more information if needed. Successfull delivery may potentially lead to several other project assignments.