We use Royal Caribbean, and they have a pretty messed up system, but if you are smart you can take advantage of it. They change their excursion and add-on prices constantly for the few months before you set sail. They do allow you to unbook and rebook for cheaper price at will though. What I hate though is how deceptive they are, when you book your cruise a package like the Jet Ski might say $75 which they claim is 60% off, but then later it will be $80 and they claim is 80% off.