Credits system API / SaaS

Hi! I am looking for a credits system service. You know those "Buy 10 credits to use such service" well, I wonder if anyone has created a SaaS that solves the pain to manage credit balances. I know you can get a simple one going quite easy, but if you want to have credits that expire, credits that are added monthly, allow users to top up, give credits away, etc it can get quite complicated easily. Any ideas?