Sorry for the blatant plug but don't forget about our services at www.lagniappe-belize.com.
You can pre-order your groceries etc. from home and have everything delivered in time for your arrival. That'll save you several trips and a half day running from store to store to find what you need. We figure it's better to spend your vacation time fishing, diving, lounging and exploring than grocery shopping.
We take VISA and MC, AMEX will come shortly.