Amazon has a targeted offer for Prime members who make their first Amazon gift card purchase. You can use promo code 18PRIMEGC to get $15 in Amazon credits when you make a purchase of $50 or more in Amazon gift cards. The offer could also be targeted, besides being limited to Prime and first time gift card buyers. Worth a test anyway. It didn’t work for me. I got “Unfortunately, you are not eligible for this offer.”
Let me know if it works for you!