My first deal post, sorry if I mess something up. I searched and didn't find this one posted yet. Just saw it a few minutes ago on Amazon's site. It does say to eligible customers, so not all may get the offer. I got the offer and it worked for the credit.
Link: Amazon Credit with Gift Card purchase offer [amazon.com]
Details from Amazon site:
Enjoy this exclusive Gift Cards offer: Surprise someone with an Amazon.com Gift Card, and get a $15 promotional code to spend on yourself. Just purchase at least $50 in Amazon.com Gift Cards in a single order by October 31, 2017, and receive a $15 promotional credit automatically applied to your account after the qualifying order has shipped. An email notification will also be sent within three (3) days after shipping, confirming your Amazon.com Promotional Credit.
Enter "OCT17PRIMEGC" at checkout or click the button below to qualify
Important Notification: This offer is valid ONLY to eligible customers. Terms and Conditions apply; scroll down to learn more.