So is Mexico. it's still an international destination.
While items manufactured in the USA (and Mexico and Chile) are not dutiable under NAFTA, they are still liable for sales taxes. The shipper will also charge a service fee. ($5 approx for Pitney Bowes/GSP, $10 for Canada Post, $25 and up for couriers like UPS).
Items sold in the US but manufactured elsewhere are liable for duty.
Here is the Canada Border Services website.
http://www.cbsa-asfc.gc.ca/mobile/est-cal-eng.html
Some US sellers do not realize that there is a different and cheaper rate card for shipping to Canada. A polite note before bidding can help the seller adjust his pricing and improve his Canadian sales.
Many US sellers do not realize that the Global Shipping Program should not be applied to item selling under $50. Again, a polite note helps both seller and buyer.
Politely-- Captain Malcolm Reynolds