I don't think Trip may have completely understood your post.
Carnival is not the "busses of the sea." It is a real and quite good cruise line. There are some day cruises that fit the "busses of the sea" profile but Carnival does not.
Having said that; I would check into EVERYTHING you will have to pay in order to receive this free cruise. My guess is that you will be required to pay the port charges, all taxes, including the airline taxes and you will end up with a three or four day cruise in the lowest category cabin and you will need to cruise on a few specific days. This will probably cost you a minimum of few hundred dollars and far from free. You will also have to sit through the time share, MLM or other pitch and the high pressure tactics associated with it. This alone would make me throw the invitation in the garbage.
There is rarely a "free" cruise.