there is no such thing as overpayment
since you have 3 gift cards you will be making 3 separate transactions to pay off the balance
transactions 1 and 2 will use up both 500 dollar gift cards
transactions 3 you will only use up 379.62 out of 500 dollars on that gift card. The remaining 120.38 stays on the card
you can use the remaining gift card balance for other things pre cruise like transfers, shore excursion, wifi, etc, etc.
If you want to use the remaining balance for things onboard the ship then bring the card or the card information with you. use the kiosk or guest services to add the gift card to your onboard foilo account