Last year I brought a duffle bag of school supplies, art supplies, etc. down for Be Kind Belize and I contacted the Ministry of Finance.
Had to supply them with an itemized list with costs of what I was bringing down and explained who it was for, etc. They sent me a letter and customs was a breeze.
I think I still have the name of the person I contacted. Feel free to PM for the info if you want to go that route.
FYI: It did take me about 2 months of e-mailing back and forth from when I originally requested the letter to when I actually recieved it in the mail.
http://www.customs.gov.bz/exemptions.html