No, its quite obvious that you need to opt out if you don't want to pay -- they explicitly say the membership will be billed to the credit card attached to your account at the end of the free period, and force you to attach a credit card to your account if you don't have one already attached in order to sign up for the free Prime trial -- when you sign up, though its also easy to forget afterwards. But they give you a complete refund anytime in the paid period if you ask, as long as you haven't used any Prime-exclusive benefits during the paid period.

And its a zero-friction, click-a-button, cancel-and-get-refund thing.