Amazon, like so many dodgy sites, deliberately try to mislead you when going through the payment process, by highlighting the button where they want you to go, with the normal option which you require in ordinary text, often well out of the way. So it's definitely a case of Caveat Emptor - Buyer Beware.
Here is their Help Screen to cancel :-
To cancel your Prime membership you can follow the easy steps below:
Go to Manage Your Prime membership.
Click Manage Membership located at the top of the page, and then End membership and benefits
In the cancellation page, you can click End my benefits or Continue to cancel.
Paid members who haven't used their Amazon Prime benefits, including One-Day Delivery, Prime Video, Music Prime and the Kindle Owners' Lending Library will be eligible for a full refund. Paid members who have used their Amazon Prime benefits may be eligible for a partial refund based on use.