Implementing coupon system for saas product

This feels similar to my question

I will also like to hear what solutions exist, but until then I can elaborate on my first attempt -
After a very short hunt in Google I gave up and just implemented some no-sql DB doc structure to keep track per user:

1. How many credits are available for the user
2. What features are enabled
3. What promo-codes (vouchers) were already used by the user

It didn't take much to implement, however it does feel like a design burden - because now we probably want to expand the most basic solution and maybe change nCredits to a dictionary of different credits types. Also we probably want to keep track of the events of activation / purchase / expiration of vouchers and it can escalate quickly.

My guess is that most products do implement this from scratch, and after an initial period with real users, it can be limited to a small enough scope and maintain a custom implementation that is tailored to the product.

We use Stripe and their tools for coupons and per-customer credits. They have a pretty decent system that accommodates percentage and currency value coupons, with expiry dates and max number of uses too.