I went on that itinerary earlier this year on the Voyager of the Seas, and the RCI shopping experts on board (husband and wife team) recommended that Cozumel was the best for shopping of the 4 ports. I did not shop, but the biggest shopping port talk on the TVs was Cozumel.
Cozumel is rather shopper-friendly, with most of the major shops clustered together and easily walkable. Shops range from large and airy to tiny mom & pop operations. There is some hustle going on (trying to get people into their stores), which one should expect in the Caribbean and Mexico, but not too bad, and a wide range of craft and souvenir items that are fun to look at. I can't answer for the other ports on your itinerary, as I haven't been there yet, but I think you might enjoy Cozumel's shopping. Prices should be better than on Grand Cayman too.
It depends on what you are shopping for. If there is something that you looking for check out the prices at home first or on the internet. The last cruise we were on in January any savings we had was wiped out by the duty we had to pay. Yes; you are allowed some exemption but if you go over you pay. Customs is really cracking down. If you are just buying some cheap souvenirs anywhere is a good place. You start out at $200 haggle down to $50 buy it and a few months later you put it in a garage sale for $1.00. Have fun and enjoy your cruise.
Costa Maya is expensive well except for the Hard Rock cafe shirt I picked up for $3.00, A co worker returned from Costa Maya and said he hated it there and never wanted to go back so when I went 4 months later I found this hard rock shirt in a t shirt shop and bought it when I returned to work he said how was Costa Maya I said boring until we found the Hard Rock then we partied all day he looked at my t shirt and questioned me for a week where the hard rock was at, man that was a fun joke. but in all serious, Costa Maya beach day, Cozumel walk 2 or 3 blocks off the tourist area and you can get some good deals not much as their are more and more tourist going 2 or 3 blocks in town looking for the good deal, rule of thumb if the have no electric a deal can be made. Belize and Grand Cayman I have not been to.
__________________ Fantasy ~ 10/01 S/S Norway ~ 04/02 Jubilee ~ 11/02 Ecstasy ~ 04/03 Norwegian Sky ~ 09/03 Splendour of the Seas ~ 04/04 Rhapsody of the Seas ~ 10/04