My own experience, and I mail hundreds of small packages to Belize each year, and get quite a few from Belize as well, is that mail service to and from Belize City is fantastic -- I can send or receive Belize City mail faster than mail from my home in North Carolina to Boston or even Atlanta. Only once in a blue moon is a letter or package lost. Belize's mail service is FAR superior to that in Mexico, Costa Rica, Honduras or elsewhere in the region.
Now, to San Pedro it takes a little longer going and coming, but only a few more days.
To more remote parts of Belize service can be very s . . l .. o .. w. Sometimes it takes months to get an airmail package to Placencia, or from there.
The downside is that postage to Belize from the U.S. is very high, and there is no longer an international book rate, so books and magazines, which are heavy, are very costly to ship.
Belize First Magazine http://www.belizefirst.com/