New subscriptions have the option of purchasing Pay As You Go credits. These credits last for 12 months, and will be used if you run out of credits from your monthly quota.
Pay As You Go is not available on legacy subscriptions. If you run out of credits on a legacy Starter plan, you’ll need to wait until your current billing cycle renews for your credits to reset. Alternatively, you can upgrade to a higher plan at any time. You can also set your subscription to cancel - when the cancellation takes effect at the end of your current billing cycle, you'll be able to resubscribe to our new Starter subscription with the option of Pay As You Go.
On legacy Creator, Pro, Scale, and Business plans, you can enable usage based billing. This allows you to pay for additional credits once you've used all of your included monthly credits. Usage based billing is typically charged at the end of each billing cycle. However, if your usage based charges exceed twice your monthly subscription fee, we will charge you immediately for the outstanding balance.
If you decide to upgrade or downgrade your subscription while having an outstanding usage based billing balance, both the balance and the new subscription plan price will be charged at that time.
For more information, see What is usage based billing?