Customs fees Canada = > USA

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.

Please make sure that you are not confusing the terms used when importing goods.  Good shipped from the USA to Canada will be subject to Import/Customs duties which are based on both the origin  of manufacture and the good's general classification.  Goods manufactured in Canada/USA/Mexico are not subject to Import/Customs duties.  Additional fees include Harmonized Sales Tax (HST) which is based on the value of the item being imported.  HST varies by province.  The last item will be a fee charged to clear the item through Customs (Customs Clearance or Customs Brokerage fee) which will vary depending on the service used to clear customs. 

I’ve had items (cables, roller blocks and platforms) shipped to me from Canada and I’ve shipped into Canada via USPS and never had Customs fees applied. Shipping via Canada post the same. 

If your sending them back to the manufacturer and they will in turn send them back to you after, there should be no duties what's so ever, you are essentially getting it serviced. Also as a Canadian who deals often enough with the USA never send items across the border via FedX always use USA-Canada postal service if you can they will not add extra fees unless they are warranted.  Adding to that always send them priority post that was they clear customs faster and with less headaches, also always have insurance for the proper amount. you can also claim in the forms that it is being returned for service.