Liquor in BVI seemed to be either the same or often cheaper than US prices when we went last year. Consider Caribbean Cellars also, they had good selection and communicated well with questions and substitution needs. We will definitely use them again. Beer prices are definitely on the pricey side, but if I were in your situation I’d be considering the hassle of having to lug around all the booze you’d need to transport for a charter trip versus the price of spending a little more to just have it delivered to your boat and be done with it. If your group is splitting the cost I can’t imagine it would cost each person too much more to just buy it in Tortola and save some effort!