You could call Mixology on St. John @ 340 714-5985. With an order that size they might very well deliver it for you to the Westin.
Prices change all the time. But I can tell you that I didn't find Cruzan Rum any cheaper anywhere on St. Thomas than I did at Mixology back in January.
Basically all the car rentals in St. John offer equivalent size cars for the same price. I always rent thru Conrad Sutton for no other reason that you can leave your rental in their lot when in Cruz Bay (parking spots are rare), and I like the challenge sometimes presented when dealing with the Suttons. If you are visiting in the summer or fall, Sutton is offering a 5% discount if you reserve thru their website (www.conradcars.com).
Your other option is to rent the car in St. Thomas and take the car ferry over. It is a cheaper alternative. However the down side is the very limited supply of 4-wheel drive rentals (which many, including me, view as a requirement to navigate some of the roads in St. John). Also, if you have any trouble with the rental you may find yourself waiting a long time for assistance.