Unless I am mistaken, this is also what Chargify is trying to solve. When I built my first app, I thought about spinning off the billing layer into a separate service, but then realized that established companies like Auth.net have all of the real leverage. We'll see how this plays out in the long run, but I have a feeling that these types of services are in for a bumpy ride as their customer base is probably the least stable of them all (start ups).