dspdave,
Last time we were in Cozumel, DH and I bought matching Diamond wedding rings at Diamond Internation. We had been looking at home and didn't find what we wanted. Their selection was very good and so was their price.
We have also shoped at DI in St Maartin and found them to be very reliable. DH wanted a garnet ring, which we couldn't find anywhere. They showed us what settings they had, that a garnet could be put in. He found what he wanted and they shipped it to us, no problems.
My only suggestion is to start shoping early in case you need something sized. We almost missed the ship while waiting for our rings to be sized.
sue