App: Charge based on usage

[cerz01]

I have developed an app that charges the user based on usage, but I don't know how to charge the cost after it has reached the preset value, since the only way I know is through the admin auth in the loader as soon as the app is loaded, but I can't do it like that because otherwise the charge only depends on whether the user will enter or not before the end of the month, so my question is if there is a way to charge the cost of the app through an external private server, can you tell me how to do it?