Simon,
Take a look at these three threads for more info on why those two payment services for ecommerce were chosen.
https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=000239&topic_id=12&topic=OpenACS%204%2e0%20Design
https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=0002TD&topic_id=11&topic=OpenACS
https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=00016C