Sorry you had a negative experience. I have used Royals and Carnivals gift card. They do have different guidelines for use. I used Royals most recently and could use them for payment up until final payment. I did not attempt to  book any excursions, ...

Goodness! Well said. I have tried to purchase and my sale would not go through, thought it was just me, and even the weather issues. Sorry to hear of all of your issues but glad to know, it was not the operator error (this time). 

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...