Even shipping commercial products to Canada via UPS and other carriers can be a nightmare for smaller businesses - but I have no doubt the big players get all the NAFTA benefits with no problem, as the treaty's authors intended. I know of several decent-sized firms in our industry who have simply stopped trying to sell to Canada because they can't find any way to make money doing so.
Meanwhile, we seem to let Canadian firms export whatever they want to the USA. In my experience, Canada is just as big a part of NAFTA's unfairness as Mexico. They have both rigged the system so they get all the benefits and the USA (outside of politically connected major players) gets none.
Personally, I would think it would be extremely difficult for U.S. firms to sell almost anything north of the border for several reasons, including a small customer base up there, a weak Canadian currency, and low disposable income among most potential customers up there.