Technically you are allowed to bring back to the U.S. only one bottle of liquor per adult duty-free. After that, you are supposed to pay duty, typically about 10% of retail value. As a practical matter, customs rarely enforces this. Guess it is hardly worth the time and trouble to collect a couple of bucks from tourists who are bringing back a bottle or two for personal consumption.
What I do, if I'm bringing back more than one bottle, is just to note under purchases on the back of the customs declaration form that I have purchased liquor and give the approximate total price, along with whatever else I've purchased. This way, I'm clean on the declaration and it's up to customs to tell me if I owe duty or not. On scores of trips from Belize and the Caribbean, I don't think I've ever been asked to pay duty on extra booze.