To the people having trouble with the Audible/Amazon local promotion: Are you entering the voucher code in the gift code or a promo code?
I didn't have any issues with it, but I entered it as a promo code on the
Redeem Promotion page you might try there if you haven't already. Just a thought. I don't have a membership with credits so that may also make a difference.
P.S. It will take you through to "Welcome to Audible" page with another button to click in order to claim the voucher if it works.