You scared me for a moment as I made this mistake years ago, buying iTunes gift cards to buy a new phone.
iTunes gift cards could only be used to buy stuff in iTunes and later the app store.
They had a second "Apple Store" gift card that could be used for physical items.
The good news it looks like they just have one card now that can be used for everything.
Do you know if there are limits when buying through
apple.com with gift cards? Will you be able to use 12+ cards at once to buy that new phone?