Any B&M store with decent POS software can accept more than one payment method on the same purchase. For online purchases, you'd probably have to contact customer support (of the store, not the card), as most online sales systems are only designed to take a single payment method.
Just remember to keep track of your balance. These gift cards act as credit cards. If there's only $4.97 left on the card, and you make a purchase for $5, you need to tell the salesperson the exact amount to put on the card. While non-CC gift cards (the ones that you can only use at the store you buy them at) can have the remaining balance queried automatically by the POS software, that $5 purchase will automatically take the maximum it can from the card, and then show that you still owe $0.03. The credit card gift cards, however, being a credit card, cannot be automatically queried for the balance (if they could, it would make CC theft so much easier, anybody could just query the balance, find your credit limit, and run it up just below your limit to have less chance of the card being locked out). This is why you need to tell them the exact balance if it is less than the total purchase price. Usually there is a number on the back of the card, where you just call, type in the CC number on the card, and get your balance.
What a joke they are, for each card there's a $5.95 service fee and if I don't use them in the 1st 12 months there's a extra $2 fee per card.
AFAIK, that service fee should only be for the original purchase of the card itself. If the cards were given to you, that fee should've already been paid. And, as jason4207 said, it shouldn't be hard at all to use them up on groceries before the 12 months is out.
i noticed alot of companies are going towards this prepaid deals. they're relying on people not using the leftovers.
afaik verizon is a company doing this. your rebates are sent in a prepaid visa.
The company that sent you the card wouldn't make anything on "leftover" balance. Once the card is active, the CC company is holding the money. If you don't use it for a year, and they start charging the usual $2 per month after 12 months of inactivity, it is the CC oompany keeping those charges, not the rebater.