Hyde do a great job for larger deliveries but if you are looking for regular small shipments then I have to recommend Shipito - http://bitz.bz/postit
Everything is handled online, you get a choice of US addresses (one in Oregon which is US sales tax exempt), you can select parcels for consolidation, choose your shipping option and track it online, right down to delivery at the local post office. I say post office as USPS Express post to Belize is pretty amazing right now (fast, cheap, reliable and right to the Caye).
You can choose a "pay as you go" type plan or if you are a frequent shopper/shipper (who me?!) then you can pay an annual fee for a virtual mailbox.