Buying your airline tickets to meet the start of a cruise has always been a bit of a losing proposition. Booking through the cruise line is often the only way to ensure that if your flight is late and you miss the sailing, you won't have to bear the cost of meeting the ship en route. Plus, it's just easier to ask the cruise line to take care of your whole vacation package.
The problem is that booking airline tickets yourself has usually been cheaper. Especially in the age of lower-cost ticketing options, you didn't always get the cheapest deal with the cruise line. You traded cost for convenience.
Now Royal Caribbean is so confident that it can deliver the cheapest airfare in conjunction with the beginning and end of your cruise that it is offering a money-back guarantee. If you find the same flights for less within 24 hours of booking (so you can't wait until you spot a sale and say, "Gotcha!"), the cruise line will refund 110% of the difference in the form of an on-board credit to use the way you like.
Granted, that's not going to be much. A $10 price difference would get you $11 in on-board credits. That's like two beers. Plus, you have to spot the lower price yourself and report it. But still, it is a sign that cruise lines are addressing the perception that buying airfare directly through them is a money drain.
To qualify, bookings must be made through Royal Caribbean's ChoiceAir system. To promote the guarantee, the cruise line will give a $50 on-board credit to people who book cruises through (but not necessarily to travel by) July 14; use SOX9 as the code offere to get it.