As for North American (including Mexico) made or not. Even if an item is "made in America" not all the components necessarily are. For eg: While a Viewsonic LCD Monitor may be American, I bet a lot of the chips etc that go into it are made overseas and are taxed as such.
Another thing that the US is very strict on is what types of items get into their county. I remember a few years ago when I was in the Trade Show biz a customer of mine wanted to give away canvas bags with their logo on it. Well the entire shipment (booth included) got held up at US customs because it just happened that the US had hit it's limit on imported textiles. We had a choice: unload the bags and send the booth on it's way, or pay a fee equal to what it would have cost to have the bags produced in the US.
I have to assume that Canada has similar restrictions.