I have a service which is paid per unit per month e.g. a monitoring service which charges $1 per monitor per month. Users can add or remove monitors at any time, and are charged at the end of the month, based on the maximum number of monitors they had active during the month. The number of units per month range from 1 to about 100.
I don't see any in-app purchase types that suit this method. I could possibly set up a hundred different levels and users select one based on their expected usage, but that will surely end up with some people using less than they paid for and expecting refunds. Or running out in the middle of a month, again causing problems.
Any ideas on how I can handle this?