I would estimate 20%+ if you go to Cost-U-Less for bulk purchases and then to Plaza Extra for the other items. Both are competitive on booze depending on what's on sale that day. In November, Plaza Extra had Carib cans onsale for ~$19. I also got a 5% discount on the total provisioning purchase of $1000+ just by asking the store manager. These two stores are only a few minutes apart.

If you go this route, anchor or grab a mooring ball near Frenchtown Marina at west end of Charlotte Amalie harbor (check with CYOA on mooring availabilty), dinghy to the marina, then walk the 150 yards to pick up your rental car (that you reserved in advance) from the Avis counter at the seaplane base. A 24 hr minivan rental will cost about $70 (<$50 for a sedan) with pickup/dropoff at this location - more flexible and less expensive than a roundtrip taxi.