I fly 50K a year between STX (St Croix) and DCA through MIA. Just like any other TSA facility, you will not be allowed to bring alcohol onto the plane as a carry-on in STT. You can check it as baggage (6 bottles duty-free). The US Virgin Islands are a port of entry into the US Mainland. You will have to fill out the standard Customs form just as if you flew in from London. Bring your passport. You can get through without one, but CBP will ask you lots of questions.