I highly recommend you have a guy in STT. I use Stanley at Love cab. He will pick you up at the airport and get you to the best ferry option. Charges basically the same as the cabs and often can get a ferry held if needed. If you are missing bags Stanley will make sure they get on the ferry to the BVI. Keep one thing in mind, the chances of having a late bag go up exponentially with when you check the bag at the airport. Try hard to have the bag checked 90 minutes prior. Once you check the bag the airline delivers it to TSA. BVI bound bags often have things in them TSA wants to look at. Once they have ogled your bag it is released back to the airline to transport to the plane. If you check it 45 minutes prior it might not make the flight.
I recommend the same about having a guy in the BVI. I use Frank Ferrol and again he will coordinate getting a late bag to you and meet you at customs to go to your charter company. He will run you where you need to go for errands at a reasonable price.

Stanley-Love cab-STT
340 473 6344
Frank Ferrol-BVI
284 443-5654