One of the pilots in the 14th MAS brought a whole Honda 250 back to the states duty free by bringing it back part by part. He stashed the parts at Yokota, and he'd bring back a wheel one trip, a frame the next, an engine the next, etc.
I was on the Thai Shuttle mission with a senior 14th MAS Loadmaster as part of the crew. This Load was lugging two huge cartons ever since leaving U-Tapao heading back to Norton.
While clearing customs at Hickam, the inspector asked him to open the cartons, which he had declared "for personal use". In the two boxes were 850 elephant skin billfolds. Customs promptly seized the billfolds. He subsequently appealed the confiscation and successfully retrieved his billfolds. He was never able to live that one down.