IMPORTANT PSA: I would suggest everyone who has an audible membership or a free trial to get on this week as there's a pretty great summer 2x1 sale, with a wide selection of titles. I've already spent the six credits I had saved, and thinking of buying a couple extra ones, especially considering some of the books on sale include complete collections and full cast readings, and you can get two of those (days worth of content) for a single audible credit. Sale ends this Sunday.
Pro-tip if you're interested in the free trial: look for referral links that say you can get 2 free books instead of just one (which in this case would translate to four free books if you use them on the 2x1 sale). You can still cancel before your free month is up and keep the books, or modify your subscription so you only pay for a single credit a month at a pretty reasonable rate. Just be warned, even though audible is an Amazon company, you need to use a credit card for all your transactions, and you can't use Amazon gift cards or credit you get back if you're refunded for items sold through Amazon.
You can still buy audible audiobooks through the Amazon main site without a membership, but they are each sold at different rates (just like Kindle books, depending on length, date of publication, popularity, etc), but they're 10% cheaper for members. So if you're interested in an audiobook that's normally priced at $5.50 (like novellas or collections of short stories), you can get it at $5 through audible while your membership is active, without having to spend a $15 credit for which you can get an audiobook priced at $75, such as the complete collection of Sherlock Holmes novels and short stories read by Stephen Fry.