We have used USPS.
Two levels of service.
GLOBAL EXPRESS gets here in 3-7 days (maybe....)
GLOBAL PRIORITY gets here in 10-13 days (maybe.....).
It usually works fine, sometimes takes a little longer.
Post office here adds duty when you pick it up. Sometimes they open the package and make sure that it holds what is claimed on the outside of the package. I have had them re-value a Christmas present because it wasn't claimed as high enough by the person that shipped it.
Some stores will ship vis USPS and add the freight charges to your order. Sometimes we have to send it to a friend and they reship it.
Anything shipped here by USPS has to have a customs form attached where the contents are listed and the value claimed.
Avoid DHL as a shipment automatically gets around $70 BZ added to the freight cost to get it from Belize City to San Pedro.