Mike, I'd suggest contacintg US Customs and also contacting your local federal reps, i.e congresscritter's office. The smart ones will make a phone
call to help a constituent who has a problem with federal bureaucracy. "Hello, this is Congressman Whoever's office, yeah, you're holding up a spare part for one of my voters, he's just trying to fix a boat." Sometimes that's all it takes.
Next time around, work with the sender before they fill out any paperwork. Often something generic and safe like "machine parts" or "farm equipment" or "trade samples" will go right through, while anything more specific that MIGHT be a problem, gets held up.
A friend of mine does customs paperwork in an entirely different field, he's learned that it doesn't matter what you are shipping--if you use the wrong word, it gets stopped or tariffed. Use another word (no lying needed) and it sails
right through. Now...you may need divine intervention, which is what the CongressCritter can provide.