I pose this question as something I am struggling with currently.
Though we offer a robust SOAP/XML based API, as well as a cool AJAX based Simple API ordering widget, I'm finding the fact that neither of these items has modules and/or plug ins to any of the Billing platforms that most hosting companies use quite a roadblock.
Is that a deal breaker for you when it comes to adding a new product offering? Do any of the currently available platforms allow you to customize something like an API into them, or do they have to be proprietary from the billing manufacturer?
Any feedback would be most appreciated!