Real world examples with billing 

If I have an invoices app so :
Plan 1 is 10$ with 30 invoices
Plan 2 20$ with 100 invoices
Each extra invoice is 0.1$

how can i handle billing here with renew
And changePlan

also when canceling plan the user should still hVe access until the period is over.

Also,
What about special plan for specific customers?

Thanks!