Be careful. I recently purchased something from the online Apple Store, with a small balance to be charged to my credit card. The order went through fine, and they shipped my order.
*After* I received the goods, I start getting emails from Apple stating that one of the gift cards was "used up". They asked if they could charge the difference to my credit card. I said "NO". I explained that I have a receipt indicating everything was kosher with the original order, I knew for a fact that all of the gift cards were fully charged and unused, and asked Apple to investigate further. Apple store rep said that they would do so. Two days later, I see a charge on my credit card for something resembling (but not exactly) the contested difference. I took it up with my credit card company. Apple completely fumbled the charges to the credit card and to the various gift cards. The amounts were crazy. Long story short, I got my money back from the credit card company.
From my comparison of my receipt at the time of order, to the receipt generated after I got Apple's emails, it looked like a bug in their online store software that happens when you mix a bunch of gift cards with a credit card.
I don't know if this would happen with an in-store purchase. The online Apple rep said that if a gift card was used "recently" at a brick-and-mortar Apple store, and then used at the online store, it would NOT correctly reflect the balance on the card. The systems are not linked in real-time. How broken!