I actually found that Hermes was more expensive in St Barth. We go there every year, and each year, I check in to see what they have. I never purchased anything (although I am still waiting for the St Barth edition scarf to be there when I am!) because they charge in euro and with the horrible exchange rate, it comes out to be more expensive than in the US
I find cartier to be less expensive because they charge in dollars for US visitors and there is no tax.
Not sure about anything else although there is other great shopping both in Gustavia and outside (near Lorient beach). Please go to Lili Belle. Its amazing for clothing!
I am so jealous-I have to wait until April to return
Have a fantastic trip and if you need any other recommendations as to restaurants, beaches or shops, PM me. I love talking about St Barth