There's also some shopping on St John, but definitely not like what' on St. THomas. However, there is a shopping area right by the pier in St. Thomas (Havensight Mall, I believe). It's got virtually all the same stores you would find in the downtown area and much more convenient and quick to get through. It doesn't have the same "charm" as shopping downtown, but if your wife's goal is to get a good bargain or just to SHOP, then that would definitely save you some time.
By the way, we walked to the downtown ferry area from the pier and asked a couple of guys sitting on a bench where to wait for the ferry. They said it was the dock right near where they were sitting. A little later we realized these guys were taxi drivers and the actual ferry dock was a couple of hundred yards further down toward town. As soon as the ferry pulled out they offered to drive us to the other ferry dock at the end of the island. Needless to say, we declined and went and found another cab driver who drove us out there. Make sure you allow time for getting to the ferry a bit early and tehn waiting for it to depart. I would say you could comfortably get from your cabin to the ferry to St. John, to the beach (via another taxi), do the beach, taxi back to the dock in St. John, ferry back to St. Thomas, and taxi back to the pier in 5 hours. I wouldn't allow less. You should be able to shop in 2 hours (5 minutes if it was me). So if you have 7 hours in port, you should be OK.