Amazon currently has a promotion going on where if you purchase $50 of Amazon Gift cards you’ll automatically receive a $15 promotional credit to apply to your Amazon account. Unfortunately though, this is a targeted offer, but you might as well check and see if you are eligible!
You can see if you are eligible by signing into your Amazon account and seeing if the offer pops up for you on this promotion page. Or, during checkout, apply code 2017AMZFAM and see if it takes. This offer expires December 31, 2017.