Are you sailing by yourself? I checked the sky auction website (mainly out of curiosity) and noticed that in the "buy it now" cruises, the small print stated that the prices were based on double occupancy. Meaning that the price you thought you bought the cruise for, did not include the single supplement charge. The amounts you are speaking of sound like they could be that. But like the others have said, we need more information. What does the "amount due" line say? If it says $0, then no, the cruise linel is not going to come back and ask for more money.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007