Confused? You will be.
I actually had the reverse experience. I paid for an item and thought "Wait -- did I just sign up to a 'free' trial of Amazon Prime." So I investigated all the available settings in my account, and it seemed that I hadn't. But anyway, I emailed Customer Service to confirm, and had a reply! confirming that I hadn't signed up.
I mostly don't use Amazon now because of their tax dodging; and also because their seach functionality is shit.