There is no duty on car parts at the border, since Free Trade. However, Canadian Customs will collect 13% HST on goods, after adjustment of value from U.S. $ to CAN. $. I am about 1 1/2 hours from Port Huron, and have a parcel drop there. My son is only 1/2 hour away so he sometimes picks my stuff up for me. At the customs inspector's discretion, he usually lets small purchases go with no HST charged (usually less than $200). Just don't try to BS them and have proper invoices with you.
I think people are are getting having to pay brokerage on their shipment confused with paying duty. Most carriers are not bonded companies, and have to hire a broker to do import paperwork, and this charge is passed on to the customer. I try to have smaller parts shipped USPS (not UPS) to avoid brokerage charges, because United States Postal Service is a bonded carrier (as is Canada Post). I like ordering from RockAuto, because their site recognizes that I am ordering from Canada, and lists the prices in Canadian $. They figure out the HST charge, and adds it to the total, along with shipping charge. This number is then translated back to U.S. $ at checkout. My shipment arrived at my door with no additional brokerage charge. As a general rule, I look at the U.S. price of a part I need and simply double it to get the approximate Can. price, delivered to my door. In other words, a $100.00 U.S. part will cost me about $200.00 Can. by the time I get it to my door. This includes original cost, HST, shipping, and brokerage.