Car parts are not viewed as personal items. When I shipped a bunch of FD RX-7 parts back with my personal goods like Kitchen Wares, Bicycle, Clothing, Books etc.. I had to pay a customs fee on the RX-7 parts I brought back as they were not considered 'personal effects'. I marked them down in price on the customs form though to keep it all under $1000 and with a bit of creativity was able to find equivalent parts on eBay that were cheap to confirm I was being honest. (hint the word was equivalent) but I still had to pay a fee. Even though some of the parts were never purchased (like the seats) I had simply removed them from my car and put in the OEM Recaro's instead.
I was a bit ticked off about it at the time.
I have also been told that a complete motorcycle is subject to duty, but if you split the shipping into 2 (the frame and motor) separate. You can bring in the whole thing (in 2 loads) un-taxed. .
I know that when my Mikuni carbs were brought in years ago I put them down as motorcycle carbs and didn't have to pay any fee's.
Hey they could be used on a motorcycle also!