Customs fees Canada = > USA

NAFTA, the North American Free Trade Agreement, allows goods manufactured in Canada, Mexico and the United States to be shipped from any of those three countries to the others without customs duties. The key, then, is where the cables were manufactured. If they were manufactured outside those three countries customs duties would be levied if they were shipped from one NAFTA country to another.
The Trump administration negotiated a replacement for NAFTA but NAFTA is still in effect until the new agreement is ratified.

It's not surprising a FedEx agent doesn't know the rules, my experience is that even customs officers at the border don't know them very well. The regulations on imports and exports are both very complex and very difficult to understand even if you have all the official information in front of you. In addition, the rules change with every new policy shift by the federal government and those shifts happen often, especially with this administration.

It might be useful to have this link to the summary of regulations by the Customs Department on audiogon's transaction pages. 
https://www.cbp.gov/travel/international-visitors/kbyg/customs-duty-info 

But even with that information available you might want to rely on a customs broker if the transaction involves much money or an item large enough to attract attention.

The flip side is that there is so much stuff flowing back and forth that an individual with a small item usually isn't worth a border customs officer's  time to deal with it.