Have you checked https://github.com/getlago/lago ? Open source billing API for SaaS.

Disclaimer: I am one of the cofounders, but looks like this could really fit your needs, especially as you add more complex logics afterwards: prepaid/offered credits, probably discounts, etc.

Also, if it’s the first time you’re building a billing system, you might want to check this thread about billing nightmares. It seems very simple at first but it becomes messy very fast. https://news.ycombinator.com/item?id=31424450

Happy to help more (whether you use Lago or not!)!