This payment module drops into any 2.2 MS2+ codebase. It was developed and tested using the 09/07/2003 CVS snapshot.
It allows for the following:
Setting the USAePay Transaction Key
Toggling the transaction mode (test/prod)
Select which card types are accepted (Visa, MC, Amex, Disc), though the drop-down list that is presented to the user doesn't really do anything other than clue them in to which cards they can use.
This module is being used at http://buy.myxrecords.com (real store, so beware).