Hyde shipping is a freighter service that ships direct from Miami to Belize City. The nominal rate is 20 square feet or 1000 lbs for $60 usd. This gets the items to Belize City 3-4 days, and ready to clear custsoms in about 7-8 days from the time they were shipped. They ship two times per week, I think on Tue and Thur, but not sure. Check out
Hyde Shipping USPS is also a good option. It's very reliable and quick if you use USPS airmail. I wouldn't use ground, as that can sometimes take months!! It takes 6-10 days to get here using airmail, and it's usually closer to 6 days. The items come directly to San Pedro, you don't need a broker, and it's usually quicker. So there are advantages to this service, although it can be a lot more expensive then Hyde if you're shipping alot. An estimate of cost is about $100usd for 40lbs. You can contact USPS for exact rates, or check online.
There is also Sterling Freight which ships out of Houston. And there is air freight service that leaves from Miami, which is very quick and pretty reasonble price. Sterling Freight brings the items in through the western border in Cayo, and the air freight brings the items into International Airport.