I need to buy something today via iTunes. However, I have gift cards arriving tomorrow that I had initially planned on using to buy this item. If I do end up buying it today, will I be able to use the gift cards tomorrow on the balance to not have it ultimately avoid a charge on my credit card or can they not be used retroactively like this?
The TL;DR is your gift card can't be used this way.
Your account is charged before the purchase is complete. If there is store credit available on your account, your credit card will not be charged. With no available credit on your account, your credit card will be charged.