SaaS billing workflow

I have a SaaS application, I am using Chargebee.
How to set up my workflow?

Like first how to sign up user only after payment (this is not a big problem I may use an external service like zapier)

The second and most important problem, is how do I know which users are on which plan?
Plus, as you might know, Chargebee offers addons setup.

Suppose i user buys one addon (quantity 1000)
now I want to allow that user only access the resource only 1000 times (or whatever quantity he/she buys)