I tried this with Amazon bins before. I would get prime and then they would give me an extra credit, but there was a problem as soon as they realize the card doesn’t work they take away the book. They figure this out because they charge you a very, very small payment to test it. So only try this with real credit cards!
One additional method to get extra books is if you have an account buy a book and save it to your computer. Then return it. You’ll still have the book and it even still works in Itunes, but you’ll get your credit back. I like converting it with inaudible as well to make sure I have a working copy. Since I don’t know what updates they might make in the future that may cause my download not to work. Now this can’t be done too often or they’ll not let you return books anymore, but you only have to worry about that if you have an active account you use. if you use the above method this should be able to get you an extra book before you cancel. That is if they let trial accounts return. I think so, but I’m not sure. I am certain it works if you already have an Audible subscription. I’ve used it many times. A lot of times just because I didn’t like the book, but still wanted to upload it.