I have found that with the Prepaid Visa type of gift cards (the kind that are not store-specific), if I don't have enough on it to cover the whole balance, I need to know exactly what the balance is. So if my bill is $20, I could tell the gal that I want to use $17.06 from this card, and then pay the rest with cash or whatever. But if I just hand her the card without knowing the balance, they won't be able to do it.