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:
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.
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.