Well worth reading: http://canadaonline.about.com/cs/customs/a/shopshipcanada.htm
If you're on the Canadian side of the border and shopping online on U.S. sites, hidden costs may catch you by surprise. There are things you should check before you give out your credit card number.
First, check that the shopping site offers international shipping or at least shipping to Canada. There is nothing more irritating than going through an online store, filling your shopping cart and then discovering that the vendor does not ship outside the continental United States.
Shipping Charges to Canada...
Canadian Customs Duties...
Canadian Taxes on Goods Imported Into Canada...
Customs Brokers Fees...
Finally, from the Canadian government website: http://www.cbsa-asfc.gc.ca/import/postal-postale/duty-droits-eng.html