It's duty free & tax free. We have bought many pieces of jewelry over 20 years from st maarten including diamonds, gem stones & gold chains & have been happy with every purchase. Had some appraised at home in the beginning & were very happy with the appraisals & quality.
Always ask if it's their best price. It's lovely to have jewelry for special occasions. Not sure what We will buy next year for 50 year anniversary.
Congratulations on your engagement! It will be easy to find rings you love.