Digital Audiobooks are becoming more and more of a racket. Amazon recently removed the ability to buy an audio book for a $ amount. You can only buy a credit and exchange that for a book. This means that you might be buying a new $30 book for 1 credit or a $3.25 book for 1 credit.
It's incredibly frustrating and only exists to trick readers into overspending on books.
I dont actually think you are better off buying a credit in most cases. In many cases, yes. In many cases, no. If you buy older books its very often cheaper to buy the book directly.
It does look like you can buy books on audible.com still. They removed the ability from the mobile app and amazon.com. I wont be surprised if they remove it from audible.com as well.
It's incredibly frustrating and only exists to trick readers into overspending on books.