Thanks for the response Billiam. I'm thinking of buying about $2,000 worth of car\boat parts in the USA mainland and bring to some relatives in Guam.
Everything should be able to packed as checked-in luggage to Guam. If I understand you correctly, I will be need to pay some sort of customs tax when arriving at Guam regardless of whether transiting Japan or Hawaii? Bummer, since I think that if I flew California to Oregon there would be no formal customs in Oregon for that. Thanks again.