We're currently happily relying on FastSpring to handle transactions for UnityVS. As advertised, we've experienced great customer support.
Whenever you purchase UnityVS through FastSpring, it will request licenses and notify our system of different events, such as new orders for instances.
Today we're opening the code of two libraries that we've using to interact with FastSpring:
Both are licensed under the permissive MIT/X11 license. We hope you'll find them useful.