I find duties can be hit and miss sometimes but if a site has a Canadian version and charges in Canadian dollars I've found I'm usually safe even if it ships from US. I don't know if they somehow treat it as if it's still coming from Canada and the trip from US to Canada is considered as them ordering inventory or something, not sure.
With Ebay it's very hit and miss. Had to pay like $600+ in duties once and got the bill months later. That sucked. It was for a wireless microphone for my church, we thought we were saving money by buying it off Ebay... yeah we saved money until that bill came in. That was an oddball one though, normally the duties are charged at check out and are not in the 100's.