Depending on where you buy the card depends if you must pay tax. I do know the Gamestop where I live does charge tax. You may want to call the store and ask if there would be a sales tax on the card before buying it.
i been buying prepaid cards for over a year now and have never been charged tax on them so i figured that it's all included so don't worry and buy your card.
in NY State, for some unknown reason, Game Stop is the only store that charges tax on pre-paid cards. I've gotten pre-paid cards from Walgreens, Family Dollar and Dollar General and Walmart each without tax. So, I suggest going to a store other than Game Stop to get pre-paid cards. I've asked two people at two different Game Stops why this is the case, and they gave me blank stares, shrugged and each said, "I'm not in charge of it."