By George I think I've got it.
Call Carnival and explain. If they don't credit your account within say 21 days, file a dispute with your Master Card. Keep the sail & sign bill, as well as both credit card statements.
Credit card refunds can be done almost immediately by Carnival.
(cruzin' solo by design)
Freedom ('09), Imagination ('08), Victory ('08), Destiny ('08), RCCL Navigator ('08), RCCL Majesty ('08), Glory ('07), Liberty ('07), Fantasy ('07), Destiny ('06), NCL PofAm ('05), Fascination ('04), Victory ('04), Fascination ('04), RCCL Voyager ('03), Fantasy ('99)