Every plan and volume tier is available on two billing cycles. The toggle sits at the top of the plan section in Billing.
- Monthly - charged every month, credits granted at the start of each cycle, cancel any time before the next renewal.
- Annual - charged once for twelve months at a 33 percent lower effective rate, which works out to four months free. The full year of credits is granted upfront rather than metered out monthly.
The upfront grant is the underrated part. On annual Starter you receive twelve months of credits at once, so a heavy launch month is not capped at one twelfth of your allowance. You spend against the whole year in whatever rhythm your campaigns actually have.

The trade is commitment. Annual is a twelve-month term paid upfront, so you can change tier while staying annual, but you cannot drop back to monthly in the middle of a term. The monthly buttons are disabled while an annual plan is running and read 'On annual plan'. The switch back happens at renewal.
Rule of thumb: pick monthly while you are still calibrating volume, then move to annual once two or three months in a row look alike.