Just got back 3 weeks ago. Who are you chartering with? It is always much cheaper to buy you provisions at the supermarket than it is to have them supplied for you. There is a rite-way right across the street from the Sunsail/Moorings base. If you can simply get to the base mid day and stay about that night before your charter. Then you can use the time to do your shopping.
Being there in low season this year you could really see how poorly maintained things were at most places. Spanish town in particular was a real dive. Also beware, it will cost you more than $100 a night for a slip there. It is amazing what a large number of people can do to make a place more hospitable.
Bitter end was nice, and is very well maintained, as is Saba Rock. Go to the Willy T after dark. There is usualy plenty of anchoring available, so try and choose that option, otherwise it is roughly $30 a night for a mooring ball.
When you check your boat out, pay special attention to; DC electrical system (make sure all your gizmos are going to charge), A/C if equipped (make sure both unit work if on a cat), Make sure dinghy will idle after being started for more than a minute or two. Run the stove top burners as well as the oven and broiler if you plan to use them at all. Run the mainsail up the track (loose the sheet first) as a lot of times they are very stiff (had manual winches this time and it was a chore to raise the main). Open all ports and make sure they will stay open (ours continually flopped closed, had to wedge them open). Bring your own mask and snorkel at the very least. Put the dinghy in the davits at night, keeps them from growing legs and wandering off, otherwise bring a cable and padlock it to the boat at night.
Regarding the heads; make sure that you operate them both before you leave, and that they work smoothly and easily, as there is a fee if they don't when you return them. Also check for weeping type leaks at the pump. You should be able to run a dry paper towel around hte base of the pump and have it come up dry.
Be sure to hit the Rhone, the baths, bitter end, Foxys. If you plan to put those on your route, everything else will fall into your path on the way.
stock up on cash. Working ATMs were somewhat of a nuisance to find this trip (though most places accept credit cards). Don't forget it about the fee to leave the island (over $100 for a family of 5).
Finally, just get there, sit down, relax, do your shopping, get to your first anchorage, then relax some more. Enjoy.