Are you a repeat cruiser with Carnival (or any of her other cruise lines)?
Often you can get more attention for a problem by calling their repeat cruisers club.
How much is the difference? Could it be , as suggested above, that the difference is port charges or taxes or insurance? Sometimes documents will list the "list" price of the cruise. Did you actually verify that this higher amount was in fact the amount charged to your card?
Another tact is to dispute the credit card charge (in writing). They will respond to that .