I was responding to kelownagirl. Though the same holds true shipping the other way as well, since this is all due to NAFTA, which should keep people from having to pay duty on goods manufactured in north america. So duty charges would likely apply if what you are buying from Canada is not manufactured in Canada. Then there is likely a cutoff below which they wouldn't charge you a duty I would think, though not certain.

Info at the customs site, in lots of confusing government speak
http://www.cbp.gov/