What should I use as a starting point each month f

Say, I use stripe subscriptions for my users with one plan which is $10 per month for 100 API requests and that's the standard price. However, if the user has used 150 API requests, I charge them $3 more on top on $10. For 200 and more requests it's total of $17.

Say, I've subscribed the user on October 9th. On November 9th they will be charged again by Stripe. When should I "freeze" the amount of the APIs the user has used during the month to calculate the overall price and start counting from zero again for the new period of November 9th - December 9th?

As far as I'm concerned, it's either on payment.success or invoice.success event in my stripe web hook controller. However, I think it's not reliable because:

Which one is it exactly - payment.success or invoice.success - the one I need? They occur both, as far as I know sometimes payment.success occurs first, sometimes - invoice.success and it's unknown which one occurs first in each particular case.
Theres's the time period between these 2 events, I don't know how long exactly, probably a few minutes. But during those few minutes the user might consume at least one Invoice
I'm not sure if it's guaranteed that for each user the events payment.success and invoice.success occur only once per month. Do they really? If not it's even less reliable to use them to achieve my goal.
invoice.created event occurs 1 hour before the Invoice is closed.

Um. Why wouldn't it be the moment your user clicked 'BUY'? The fact it took an hour/day for payment to be received isn't relevant is it? Think about iOS apps. I can buy the app right now, but most of the time I don't get an invoice from Apple for several days (even a week sometimes).

I think you're going to be frustrated trying to time it to events from your payment provider. IMHO that should be done within your app.