From what I've seen, it's mainly prepaid. With post paid, there isn't a way to do it that I'm aware of besides buying a Visa or Mastercard prepaid card.Plus, the whole point would be to have a stable price in Monero. Those gift cards change price all the time with the fluctuation of the market. So, this would be more expensive, sure, but it would also be completely stable.Effectively acting as a price ceiling in Monero.