How does billing work when AppExchange Checkout customers add or remove licenses during the current billing period?

If a customer adds licenses during the current billing period, the licenses are available for immediate use. Checkout charges the customer a prorated amount for their next billing period. If a customer removes licenses, the reduction takes effect at the start of their next billing period. The customer can continue to use the licenses during their current billing period. Checkout charges the customer for the reduced license count starting with their next billing period.