https://www.audible.com/
$99.50 for a 1-year membership ($50 off)
receive all 12 audiobook credits up front
$50 credit towards any Echo device
You have to pay for the whole year up front, and it will auto-renew at the regular rate if you don't cancel. It looks like if you are already a member you can cancel and sign up for this deal right away.
Here is the thread on Slickdeals.