Amazon will not charge you if:

You cancel the Deal before the scheduled start time.
Amazon cancels the Deal before the scheduled start time.

You will be charged the full fee if you cancel a Deal while it is running.

You may cancel a proposed Deal at any time; however, we recommend you cancel your deal at least 25 hours before the start time to avoid paying a fee and to avoid being blacklisted from creating Deals in the future. Upcoming Deals will be visible to customers on Amazon 24 hours before the Deal is scheduled to run and can be found in the Upcoming Deals page. If you ever need to cancel your Deal, do so before it becomes visible to customers.