Hey Phil!
If you drive it to Point Roberts, you can send it UPS. It should be pretty cheap that way. But I think it´s best to package it. When I send a gearbox, I go to Crappy Tire and get a large plastic tub with lid. Take off the gearbox's nose cone and it should fit. Then after putting the lid on, drill many holes around the perimeter of the lid and zip strap the lid on. But don't zip it on until after you clear customs, they may want a look.