Educational material is not exempt from duties. I think that the basic "books/materials" rate is around 45% of value, so 50% sounds about right.
Once in while you get caught and have to pay some duty - it probably makes up for the little leeway that the customs dept have obviously afforded you in the past.
If you are desperate for some of the forms now, surely you can borrow some from a fellow instructor, and replace them when you get your package cleared.
Mr Marsden is a very fair man, but once the duty has been assessed on an item, it is not going to miraculously disappear.
Many of us have a friend bring down an item or 2 when they come down on their vacations. And whether you like to hear this or not this practice is contrary to the customs laws of Belize. All articles that are imported should be assessed and possibly customs duties would be levied.
So on the once in a while occassion when you get caught - just pay up. What are you talking about $20? $200? Whatever the amount is I wouldn't doubt that you have probably been able to smuggle in items that have exceeded that amount over the past few years.... come on, let's be honest.
Cheating/smuggling-in small items is probably the most criminal act we all tend to partake in - after speeding of course.