This has happened to me a few times in the past with Carnival after the final payment due date. Granted they have lightened up with this the last year or so. I just accepted it and enjoyed my cruise. It was the price I agreed to when I booked. I never held it against them, and have cruised many more times with them since.
Penny made a good point about this; if the price would have went up (and sometimes does) you would have been protected.