Older thread but just to arm future folks. I have a large list of $50 gift cards (Amex Platinum reimbursement anyone?) and the way that I got them to use 9 of them on a single transaction was to call the delta line and ask if they could call the delta gift card team and combine those cards into a single $450 card. Turns out they could. It wasn't easy as I wasn't allowed to email the gift card numbers and they required I read them over the phone. But after about 40 minutes they had combined 9 cards into one, and then I used that to purchase my tickets. Hopefully that helps someone.