If you try to buy free kindle books long enough you will accidentally purchase a book that is no longer free, or at a price you are unwilling to pay. The best way to prevent this is to refresh the page just before you purchase it to make sure you have to most recent purchase price.
If you do happen to buy a book that you thought was free, you can return the book within 7 days very easily.
First, you need to make sure you pay attention to your email receipts from Amazon. This is the best way to catch a book that was not free. In the past Amazon has sent you a receipt for every individual book that you purchased. Recently, Amazon has been grouping books together so you get only one receipt a day. This is much easier to look through and make sure you kindle book was actually free (or the price that you intended to pay.)
Once you see a problem, go to your Amazon Account on your computer and choose Manage Your Kindle. This will bring you to your kindle library. The library defaults to sorting by date with the most recent book at the top. If you have a large library, then it will take a while for Amazon to finish loading the whole thing. Your personal documents will show up first, then the purchased books from Amazon.
Your book should be on the first page or two since you only have 7 days to return the book. But if you do not see it, type the name of the book or author into the search field.
Once you find the book, click on the ‘Actions’ button on the Right. This will bring up a menu that allows you to push the book to a kindle, delete the title (only if you really do not want the book any more), offer it for loan, or Return for Refund.
Once you click on that another dialogue will pop up. This will ask you for your reason for return. There are a variety of options including, wrong item purchased, offensive content, DRM issues, compatibility issues, accidental purchase and unwanted purchase.
I usually choose accidental purchase and then click the Return for Refund. Usually I receive an email confirming the refund within a couple of hours. The amount is credited back to the form of payment used.
Remember you only have 7 days to return kindle purchases for refund.
You can also contact customer support and ask them to refund the purchase. But this is a much easier process than contacting customer support and it is automated, so your return is processed much quicker than going to an actual person.