It's the bane of the bargain hunter. You buy a trip for $500, and it's a good deal. You're fine with it. Curiosity prompts a search a day or two later, and the exact same trip is selling for $395.
This rather frequent occurrence has apparently ticked off enough travelers that now, many booking sites and airlines offer to refund the difference if the price of your trip decreases after you buy it -- even, sometimes, if the lower price is offered by a different site.
Sometimes. It turns out these "refunds" are almost always (or always-always) in the form of a credit for future travel. Sites that sell both air and hotel may even limit the voucher to a hotel purchase even if what you purchased was a flight. These vouchers may expire; they may come with a re-booking fee; they may require such an exact match between itineraries and purchase conditions that a reasonable person would never see a difference between the two trips [source: Airfare Watchdog].
In short, read the fine print. Twice.
Still, despite its shortcomings, for some people a price guarantee can be a very valuable thing. For instance, if you almost always fly a particular airline, and you fly several times a year, then the voucher might as well be an actual refund. You could end up paying next to nothing on top of that voucher for a future flight.
It's also, and this shouldn't be discounted, a little extra peace of mind for people who book, say, six months in advance, since it's practically guaranteed the price will decrease at least once or twice before you fly.
All of this may sound trying, and it can be, especially if finding the hands-down best deal ever isn't on your list of pastimes. But guess what: Live, breathing travel agents still exist, and their whole job is to book you the travel you want at the price you want to pay. You'll pay a fee for the service, but a travel agent can have insider connections resulting in discounts you wouldn't otherwise find. You could still end up paying less in the end.