As was mentioned earlier, RCCL and Celebrity both have very strong policies of allowing fare reductions to be passed to passengers, even after final payment, the one exception being "Happy Hour" rates (also called Sales Event on RCCL and Xciting Deals on Celebrity). These Happy Hour fares are typically on Tuesdays, though agents get access to them the previous Thursday/Friday. Usually, they are for sailings within 4-6 weeks of sailing, though I've seen some even just a few days prior.

Because you're dealing with one way TATL fares, I'd suggest finding the one you want and going ahead and booking. While you may not have the absolute best fare, that airfare last minute can be killer.

By the way, if your agent knows how to manipulate the data in the agent One Stop files correctly, and is highly familiar with it and Happy Hour trends, one can get a pretty good idea of which sailings are likely to go on Happy Hour. Feel free to send me your preferred itineraries and I can take a look for you and give you my best estimate of likelihood.