How about adding a " Downloads " system for Elk. Where we can offer soft goods and allow users to add their Paypal if they choose to receive a payment for it.
Idea: hassle the Simple Portal crew to put a download system into SP (coz TP has one so...).
Completed this request check out http://www.elkarte.net/community/index.php?topic=4221
I can probably some of the more advanced payment requests in a paid version.