We bought some of the best quality (even better than USA, IMHO) souvenier t-shirts at a little shop right in front at the dock in St. Maarten. We've had them for two years and the colors haven't faded, and they're a very good knit, 100% cotton.
Also, even though we no longer smoke (fewer and fewer do,) cigarettes are extremely cheap if you have any friends who do smoke.
My favorite purchase in St. Thomas was a beautiful knock-off Chanel purse which I still carry. It might be many years before I can afford a real 1500.00 one, and this was less than 100.00!
Both islands are great for shopping. I like eastern cruise better than the western for that very reason.
(Oh, I also heard St. Thomas is a great place to buy Wedgewood china, but a risk to ship it home if you're flying.)