When you sign up for a free trial, you need to remember to cancel your subscription otherwise, the company will likely charge you.
It’s the same thing for Amazon Prime. Many folks signed up for the 30-day free trial so they could take part in Amazon Prime Day. So, unless you want to get charged $99 make sure you cancel the subscription before the 30 days is up...which is Friday, August 11th. You probably don't want to wait until Friday morning, if you're going to cancel, you need to do it before midnight.
To cancel: log into your account and hover over your name. A list of options will pop up, which is when you’ll want to click on ‘Your Amazon Prime’. Once you do that it will bring you to a page that lets you opt out of your membership.