If you are shipping a considerable amount (12+ boxes or a small container) then ditto with Hyde Shipping and George. Duty varies by what you are shipping, anything from 5% (ceramic tiles) to 45% (cosmetics). Once this is added you pay GST at 12.5% on the full amount plus shipping, and then add environmental tax. You will also pay George a brokerage fee based on the number of "line" items.
If you are just shipping a couple of boxes, then USPS to the post office (General Delivery, San Pedro, Ambergris Caye) is the way to go. You will still pay the duty and GST but their brokerage fee is only 75BZcents per box.