I agree that Super U in Marigot has the best prices and has very large selections - mostly in French. (We accidentally bought some chicken flavored potato chips this last trip and they were actually quite tasty!) If you want a more American selection, you can go to the Maho Market by the roundabout - they've got a pretty good selection of things and the prices aren't bad (their liquor prices are excellent!). I haven't been to Carrefour yet so I can't comment but, as I recall from when it was the old store, it was a large store so I would imagine that it is well-stocked. I've read good things about their prices. Just be aware that there will be guys in there who bag your groceries and then request tips - made me uncomfortable when it was the old store.