Royal is really picky about the use of gift certificates/gift cards.
You have to book directly with Royal (after the booking is made they can transfer it to your TA for you).
You make the deposit with your credit card just like always.
Then, RIGHT THEN, you can use the gift cards/certificates to pay on your cruise.
If you have $100 cards, you can use up to 9, if you have $500 cards, same thing, up to 9 cards.
Whatever the balance is after using the cards, that cannot be paid for with cards later on. It must be paid for with your credit card before the final payment date.
The cards can NOT be used on ship.