Good question--maybe this may help. If you find a piece of jewelry that you love back home, write down the approximate price. In St. Thomas you will be overwhelmed with tooooooo many choices! My wife 'smells' sales, and you need to focus in specifics. If you feel pressured to buy something right now, you can always go down the street to find something similar (unless it's one of a kind). Generally prices that are 20% and higher (before taxes) are certainly worth considering.
In St. Martin you can find duty free goods on both the Dutch and French side. Again, if there's something specific you want (brand wise) get the price back home--local made goods/crafts are always great souvenirs and generally they're not too expensive. My wife loves spices, and there's a spice shop in most ports (just ask someone where it is). There are web sites for this as well (don't know any specivic ones, but go to Google and ask!)
Good luck and enjoy the trip.