Here's the Pertinent section of 9966.00.00
See Articles 4 & 5 which talk to parts (original or repro.)
If for some reason what you shipped isn't over 25 years old or designed for a vehicle that is over 25, then duties could be due. As a Canadian who buys things out of the U.S. on occasion, I'm disappointed that someone on this side of the border would leave you hanging like that. There needs to be trust on both sides of the border, and if someone here is betraying that, it impacts us all.
Keep in mind that one possibility is that the carrier doesn't want to go through the paperwork to clear things up with the Customs folks, so they're going after you and the recipient as that can be easier for them (esp. if they have a credit card on file) - that happened to me once when I bought a product here in Canada and the store was out of stock on one of the components ... they shipped it to me from the U.S. and I got hit with a bill for $70 for duties all because of how the shipping label was filled out.