As a Leap Day special, Audible is having a Buy One Get One Free Sale for members that have a credit on their account. It does appear to be limited to those in the US and if you have the very, very old use-it-or-lose-it 2 credits a month sale, it doesn't seem to work for you either. For the rest of us, though, we can shop as many times as we have credits stored up in the special BOGO store, getting two books for the price of one! The cart in the special store allows only two books to be added at a time, then you click the Confirm button and check out. Return to the BOGO page after checkout to repeat the process for each credit. The site is running a bit slow, but it looks like it's definitely worth the effort to explore all the categories today.
If you aren't already a member at Audible (a subscribing member, not just have a login), you won't be able to see the sale. However, you could join with the 2-Free Audiobooks Gold Listener plan and should be able to take advantage of the sale. If you decide not to keep the plan after the first 30 days, just cancel with no further obligation.