Let's say you pre-paid for 15,000 API calls per month, for the year starting in Sept, 2018 but then had 17,000 API calls in November (i.e. you went over your monthly limit).
TaxCloud's system would:
1.) Automatically bump you up to the next tier at the month-to-month rate, and...
2.) Charge the amount due against your current credit balance (i.e. your daily rate x the number of days remaining in your annual plan).