If you signed up for a free 30-day trial of Amazon (NASDAQ:AMZN) Prime during the company's Prime Day one-day sale, you have until August 14 to stop your unpaid membership from converting into a paid one. Of course, if you took advantage of the free offer in advance of the company's big summer sale, the clock may run out even sooner. 

The company has not released a total for how many people signed up for free 3-day Prime memberships as part of the online retailer's attempt at holding a Black Friday in July, but the megasale -- which broke records for the company -- was only open to to members of the company's $99 a year free two-day shipping service.

And though the company is always a bit secretive when it comes to releasing any information about how many people are Prime members, it did announce in a press release that "more new members tried Prime worldwide than any single day in Amazon history" as part of the July 15 event. For those customers, the clock is ticking, and when it runs out, they will be billed the full $99 for a one-year subscription.

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Prime offers more than just free shipping. 

How does Prime work?
When an Amazon customer signs up for a free trial of Prime, he or she must attach a credit card to his or her account, giving the company permission to charge it at the end of the trial period. In order to not be charged, the user must specifically opt out, something the company explains how to do on its website.

Go to Manage Prime Membership

To cancel an Amazon Prime free trial, click Do not continue.

Note: If you don't cancel your Amazon Prime free trial, you will be automatically upgraded to a paid membership plan at the end of the trial period. You can find this date listed on the left-hand side of the Manage Your Prime Membership page. You will still have access to your Prime benefits until this date.

It's an easy process, but one that's also easy to forget to do. If you do miss the cutoff date, it's still possible to end your membership provided you have not used any shipping benefits after the free trial period ended.

To cancel a paid Amazon Prime membership, click End membership.

Note: Paid members who haven't placed an Amazon Prime eligible order are eligible for a full refund. You can't receive a refund if any of the household members have placed an Amazon Prime eligible order.

There are no partial refunds. If you haven't used Prime to place an order, then you can get all of your money back. If you placed even a single order for a single Prime-eligible item, then you are completely out of luck.

Why is this important?
For Amazon, a large part of holding Prime day was to entice people into signing up for its membership service. Adding members is key for the company, because its roughly 41 million U.S. members spend an average of about $1,100 per year, compared to about $700 per year for non-members, according to a survey conducted by Consumer Intelligence Research Partners.

Determining whether Prime is worth it for consumers has become a little trickier now that it offers video and audio for members along with shipping benefits. On a pure free shipping basis, a customer would have to place around 25 orders to save an amount equal to the $99 membership charge. However, if that person is also watching Prime Instant Video or using Prime music, determining value becomes much trickier.

Still, whether a user keeps or cancels Prime, it's better to make an informed decision rather than simply having your free trial convert into a paid membership. If you want to make that choice, and you signed up on Prime Day, you're running out of time to do do. 

Daniel Kline has no position in any stocks mentioned. He has been a Prime member for a very long time. The Motley Fool recommends Amazon.com. The Motley Fool owns shares of Amazon.com. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.