Where does the Amazon credit show up?
The last time I did it I got an email saying the credit was added to my Amazon account (it was prime now credit). It can take up to two days:
Dear Audible Customer,
As a thank you for joining Audible today, you have received a promotional credit good towards your next Prime Now purchase.
The $15 credit may take up to 48 hours to register to your Amazon account. It will then be automatically applied to your qualifying orders in your Prime Now account. The discount will appear in the Order Summary column of the checkout page as "Promo Code”. You have 60 days to redeem. See below for additional details about this credit, as well as the offer terms and conditions.
Happy listening from Audible!