With a single account, it is possible to use a single credit an unlimited amount of times without having to go through chat/support to get the credit back.

1) Go to amazon and login with your audible username/password
2) Find the book you want and click the "Audible" purchase option
3) click the purchase option (using audible credit). Leave the tab open on the order confirmation screen as you will need this later
4) open audible in a new tab and download the book from your library
5) after the download is finished, go back to the amazon tab
6) you will see a cancel order button on the right of the page. click it.
vuoala. you have the book and you get the credit back. This will work indefinitely.

Note: Make sure you do steps 4-6 in a timely manner. if you wait too long to do these steps (maybe 15 or 20 minutes), the cancel order won't work and you'll have to use the chat to get the credit back.