Have you considered including code for linking up to popular payment gateways? You should also take a look at the SaaS starter kit for rails [1] - it seems to be similar to what you're doing and might give you some new ideas.
One possibility I thought of was adding support for payment verification callbacks. So you can create single-use digital downloads that are only authorized after payment is verified. If I ever get traction, that's definitely on my list for a future feature.
Thanks for the link to to the Saas starter kit- I'll take a look.