I've spent quite a bit of time in Belize and I have chartered out of Placencia. You can do your own provisioning if you have at least a day. There is a good grocery store (by Belize standards) and several smaller shops. If time is short, just go with the Moorings provisioning. Belize is not really set up for "one stop shopping."
Also, when you are out, there are pretty much no opportunities for reprovisioning on any of the Cayes. We chartered with a local captain and went outside the reef. He was a great asset because he bartered for fish, lobster, and conch. We even got a bucket of conch in exchange for charging a cell phone.
If you have any questions, feel free to post them here or PM me. If you are interested in spending any extra time in Belize, I can offer lots of suggestions. I have worked on archeaological projects all over Belize and spend a couple of months a year there.
"When in command, command." -- Admiral Nimitz
Difference between a power boater and a sailor out on the water: A power boater is going some place special, a sailor is already there.
s/v Zotz 1981 Pearson 365 Ketch Hull #375