Shore excursions are a major source of profit for cruise companies, which is why first-time cruisers are sometimes skeptical about booking the tours. They aren't sure they're getting a good deal.
With the guarantee, if you book your excursion pre-cruise and find the exact same shore excursion elsewhere at a lower price, you'll receive 110 percent of the difference in the form of a shipboard credit. So, for example, for a $129 shore excursion you find at $119, you'll get an $11 credit.
Cruise lines do have competition from third-party shore excursion sellers, such as Viator.com and Shoreexcursions.com and tour operators at the ports of call (some of whom provide the tours to the cruise lines).
But here's the rub: If you invoke the Princess guarantee, it has to be for the exact same outing. If the cruise line, say, adds time at the beach to a tour of ancient ruins, you can't ask for money back if you find a cheaper ruins-only tour.
If you book through a third-party company or local operator, ask about refund policies in case your ship is late into port or a tour has to be cancelled because of bad weather. In both cases, cruise lines will give you a refund -- that is where booking through the cruise line becomes particularly attractive.
The Princess guarantee of course has fine print -- namely that you have to submit paperwork at least seven days prior to your sailing date. You can't book a cheaper tour at the pier and say, "Hey, give me some money back now."