I'm not sure if some understand. It is not that you send the letter 45 days from your cruise. But when you send a letter with compliments or complaints to Carnival they normally will respond. They may respond with a letter that tells you to call withing 45 days from the sailing date of your next cruise. And that they would arrange a special gift for you. When I did it I have Birthday decorations done for a friend. They give you a list of items and may tell you to look at the web site were you can shop for gifts and such.
Hope it helps.