I have used lots of discover sharperimage rewwards cards. All of them stack neatly and have never had a problem before. I use them to buy ipod mini(s)
Just make sure you enter the nos for the gift or reward cards with the maount. They do verify the authenticity of each card used. If you have put ini gift cards/rewards cards worth more than the required amount, Si usually send you a certificate by mail for the remaining amount.