It is not at an airport, but my all-time favorite duty-free liqour store is "Al Cohen's Discount Liquor" in St. Thomas.

If you are coming on a cruise ship, it is right outside of Havensight mall, where many of the ships dock. I do not have a price list handy, but I have always found great deals here, significantly cheaper then any airport DF shop.

The best part is that since this is a US Territory, they have different custom allowances to bring liquor back to the US. The last time I checked, provided you had at least one 1 liter of liquor made in the US Virgin Islands (ie Cruzan rum), you could bring a total 6 liters back to the mainland!

I have visited three times, and brought at least 6 bottles back each time with no issues.