Price protection is the umbrella term for the programs cruise lines run to handle a fare that falls after you book. Each line has its own name for it: Royal Caribbean and Celebrity call it a Best Price Guarantee and Carnival runs Early Saver Price Protection. Princess retired its program, the Better Than Best Price Guarantee.
They share a structure. The lower fare has to match your booking exactly, it has to be publicly bookable on the cruise line's own site, and the claim has to fall inside a window that is usually anchored to your final payment date.
They differ in what they pay and for how long. Compare every line's program.
