What tour are you doing? If it's the beach & shopping tour, I'd cancel it and just do it on your own. It's cheaper in the end.
When we docked last May (07) we shopped in Havensite because it is right off of the ship. This time July (08) we ported in the Old Navy Yard. There are only about 5 shops and 4 push carts, so not much shopping can be done there. So here is my suggestion, if you want to go to the beach and then do some shopping in Havensite you would have enough time as long as you are not looking for something specific. If you have to take a cab from the navy yard to Havensite it will cost $4.00/pp. If you take a cab to Meagans bay the cab is $8.00/pp and $1.00 beach admission.
Havensite it perfect for the typical and not so typical souvenier shopping. The stores are not overpriced and everyone is soooo nice. Make sure that you take a book of shops and coupons, handed out on the port by the locals.. there are tons of really great coupons in there, especially Caribbean Hook bracelets.
I was looking for a specific piece of jewelry and ended up spending 90% of my time in Charlotte Amalie.
If you would like any more information please let me know.
-Jul