There is a big difference between freight shipping goods and bringing them in yourselves through an International Gateway, such as Intl Airport. Bringing things for schools and charities is not uncommon and the airport customs are pretty good, and fair, as long as you are up front and honest with them and have a plausible story.

When goods are freight shipped you need to use a customs broker to clear goods through customs and this is when you absolutely have to have clear invoicing that's in good order and have any relevant import permits in place. Leave any room for error and it will result in just that - you tend to learn fairly quickly and it's an ongoing process. Try to ensure your suppliers are diligent with their paperwork, get a good customs broker, build a relationship and stick with them through thick and thin - not easy to do if you are a casual shipper, so to speak. I also believe it's standard to pay Duty on the freight shipping costs to Belize - which I've never quite understood, but who says you should?

Customs will also use the internet to find retail pricing of goods and it can be very difficult to explain you bought things wholesale, or worse there was a special wholesale deal or you buy x and get y free. It's difficult for both sides too remember - I am sure some do try and cheat.