Depending on your 'needs/must haves' you may want to consider bringing some things from home. For example, while they don't travel well, potatoe chips tend to be very pricey, in Belize. Also cookies.. but there are some amazing cookie chocolate bars that are not bad in price (we got them at Island Supermarket - which does tend to be a pricy place, but super clean and more like the markets in the US). I would probably bring the cereal, especially if your gang is big cereal eaters. Coffee from the US is pricy but the local coffee is a decent price and good too..
Personally I would not worry about bringing meat.. legally you require a permit (time and money involved) and when you consider that you can eat quite cheaply there at the restaurants and street vendors or BC's bar, it's not worth it. Besides, you are in the Caribbean..
sample the fish, conch and shrimps!! (Sorry, you missed lobster season!). It is much easier these days to get decent meat in SP.
You would probably do well to support Simon's business and avoid the hassel of stocking up yourself, but as others have noted, the shopping if part of the adventure, especially if you visit the smaller Mom and Pop stores. Hey, get Simon to buy the bulk and you just window shop for yourself to enjoy the adventure. The Bakery (two locations) is a must also.. Celi's deli is great for light snack type things.. The small restaurant in Vilma Linda Plaza (can't for the life of me remember the name of it..) has good food too..
Cold drinks waiting (both adult (Belikin) and non adult (soda pops)in the fridge waiting can't be beat... :p
Whatever you decide, you won't starve and/or go without..
Have a wonderful holiday and we'll see you next year.. Gail