Ask HN: How do you keep track of your pricing plans in your SaaS?

Specifically, the interaction between a customer's current pricing plan, and the features that you enable for them. I'm trying to validate an idea of a pricing plan service that would run alongside your application that would track pricing plans, which customers are on which plan, and the features each plan enables (including tracking usage levels, e.g. the silver plan allows up to 10 messages per day and customer X has used 2 messages of out 10 today).