Two years ago, I wanted to bring down 2 duffles of school supplies and I contacted the Ministry of Finance and explained what I wanted to do. I had to send them an itemized list of everything I was bringing with costs for each item, when I was coming & if I was arriving by air or by land. They sent me a letter (with my list attached) stating that all of the items on the list were exempt from duty. I presented the letter with my bags at customs and I had no issues (I came in thru the Intl airport) and paid no duty because I had that letter.
Fair warning: It took me about 2 months from the time I started the process until I actually recieved the letter in the mail from them.
Contact:
Ministry of Finance
New Administraton Building
Ministry of Finance
Belmopan
Belize, Central America
(I do have the e-mail address for the person I dealt with. I will have to dig that up and post it as well)