Why don't you try SureRemit for such purchases? They have Amazon, Walmart, Target and close to 2000+ merchants across 45+ countries on their app. They are also launching their web based platform which will allow users to transact on the go. you can pay for these giftcards using RMT (native to SureRemit), but we are soon launching a feature where you can pay for these services using BTC, ETH, LTC and a few other prominent tokens. You don't have to worry about the authenticity as they are directly sourcing the Gift cards from the companies.