NAFTA or the North American Free Trade Act is a Multinational Treaty.
If those two words, "Multinational" & "Treaty" didn't cause you to break out in fits of hysterical laughter then please remember that this Act is hundreds of thousands of words long, written by lawyers. And that is just the first paragraph.

What it comes down to is the judgment of the individual Customs Agent on the desk at the time your package arrives at the border. Did the agent have their coffee that morning? Did they have a fight with their spouse last night? Does the Agent like the color of tape used on the package? Can the Agent read?

There are just too many variables to know for sure if your Kilt will be charged Duty during its trip.