Just tried this today with an AMEX gift card and encountered one snag, which may only apply to AMEX cards, I have no idea. It seems that when reloading the Amazon gift card balance using the remaining balance on my AMEX gift card, Amazon first made a test charge of $1. When that worked, it then made a second charge for the full amount that I had requested – the remaining balance on the card. Because of the $1 test charge, my “new” balance was now $1 less than the requested reload amount, so Amazon declined to load the card balance. Chatted with Amazon and AMEX and confirmed this scenario. It may be that the AMEX card is treated like a credit card instead of a gift or debit card and that is why Amazon insisted on a test charge, but in any case my only option would be to reload Amazon with $1 less than the balance and then when Amazon refunds the test charge, I’d be left with $1 on the card.