Are you doing this as part of a personal contruction project?
Or are you doing this as part of a construction business, where homes are constructed for profit?
It would actually be better for you, as you would be covered by the NAFTA treaty, which would allow you some special handling and treatment.
First, since this is valued above 2,000 dollars, it has to be cleared through a formal U.S. Customs broker. This is called Formal entry into the U.S.
Second, it needs a certificate of origin: http://www.cbsa-asfc.gc.ca/publications/forms-formulaires/b232-eng.pdf
If he is purcasing this through a company already operating under NAFTA, then he can be spared the fees. See this website that is of a company having an operation under NAFTA, that also sales Kit homes:
Import for the private citizen would require a 31% import tax, for this type of building materials.
however, under NAFA a builder can import without these fees.
It is cheaper if you as a builder work with the U.S. Customs agent to bring this in under NAFTA to avoid higher tarrifs.
If the individual brings it in, then he may be subject to tarrifs as high as 31%.
However, if your home owner is already purchasing this through a company that is operating under NAFTA, he may not be subject to the import duties.
This is representative company who sales kit log cabin homes: