We generally bring back a few cartons of cigarettes from SXM, always declared them and never have been charged any duty on them. We brought back 4 cartons this trip, 2 over the 'free' allowance. We declared them and the customs guy said "You know the duty is $10 per carton?" We actually didn't know that. We always thought it was 10%. It didn't matter that much, they are still cheaper, even with the duty, which is what we told him. He said OK and just let us go. We looked it up, as we thought it was a percentage, but it turns out he is right..
Duty info

Just thought I would let people know, who probably thought as we did.