It'd be fairly difficult to determine sales by cruiseline. You don't need to show your S&S [though some places ask if you're from a ship, and which one], and that'd be the only sure-fire way to tell.
The way it most likely works is a store opens up in, say, Coxen Hole Roatan, and wants to have a ready customer base of 5,000 people a week [i.e., two cruise ships' worth of passengers]; his marketing agency or bank thinks that for every 1,000 people through his store he'll sell $2,000 of stuff at a profit margin of 25% -- which is to say $500 profit/thousand customers; $2,500/week. Without the recommendation from the cruise lines, he'll get 500 people coming through each week, $1000 in sales, and $250 profit/week.
So he can easily spend $1,000/month paying Carnival to "recommend" his store; he'll still net $9,000/month. He just has to spring for one free seashell necklace to a lucky winner from each boat.
Desperately seeking 10th cruise for the free laundry.