What is wrong with Audible?

For the second time is as many weeks I have received a promotion from Audible, proceeded to take advantage of it, which really means me buying an audio book I would not normally buy, and then, upon clicking the button to complete the purchase, I am told that the promotion does not apply or that I have already used the promotion.

Here is the all too familiar error message:

Screen Shot of Audible Error

 

Update: The purchase actually went through: the audio book is now in my library. Still, Audible has some work to do. If no error occurred, then the customer should get no error prompt.