It's not opensource, but I really like a lot of the code in the RailsKit SaaS kit (http://railskits.com/saas/). If you're doing a SaaS application that integrates with payment gateways (it sounds like you are), this would be a very well-spent $250, even if only to pick and pull bits of code from it.

There are some good Rails projects with source code listed in this thread as well:

http://stackoverflow.com/questions/193632/where-can-i-find-g...

I've also worked on a large project based on the open Spree source (which was loosely based on Radiant source back in the day), but it has a lot of things that would be needlessly complex for your application in all likelihood (extensions... ugh), so that would probably confuse you more than help at this stage.