It's possible to cancel an Amazon Prime trial even after being charged for the full membership. This gives you a little leeway in case you forget when your credit card will be charged. You can terminate your membership on the website at any time.
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Removing Amazon Prime Membership
On Amazon's main page, point the cursor to "Your Account" or "Sign In" in the upper right-hand corner of the screen. A drop-down menu appears where you can click "Prime Membership." Enter your password when prompted; even if you're already signed in, you'll have to enter the password again before reaching the Prime Membership page. Click "Do Not Continue" on the left-hand sidebar to open a confirmation page. If you've already been charged for Prime but haven't taken advantage of the shipping benefits, you'll receive a full refund in three to four days.