To your original question regarding the best price guarantee, Carnival only guarantees best price if you find it within 48 hours of booking. However, as everyone else has said, if it's out of final payment you should cancel and either rebook or go with another agency.
Okay. Breathe in deeply. Hold for five seconds. Now slowly exhale. That sense of calm you're feeling is thanks to our 110% Best Price Guarantee. It works like this: you book your cruise with Carnival. And if within 48 hours you find a lower-priced fare available to the public, we'll give you 110% of the difference in onboard credit. That's what you call your classic win-win situation: peace of mind and a fruity umbrella drink in your hand.