If you plan to use multiple gift cards (more than 2) then don’t wait until the last day to do it online. Before the last day you can enter partial payments. On the last day the system forces you to make full payment and you are only allowed to enter two payment methods that must pay the full balance (2 gift cards, 1 gift card and 1 credit card, etc).
You still have the option to call and use multiple gift cards, at any time (including last day).