do you want to know what companies ship to Canada (because most do) or what companies ship that are actually WORTH shopping from? because it wouldn't make sense to pay MORE than the item itself just for shipping/customs...which is the case with a lot of the companies out there.
revolveclothing and activeendeavors top my list because they both ship to Canada and via USPS (with AE, request it because they sometimes send UPS). shopepic has decent shipping rates, too. ssense.com is also based in Montreal but carries brands like Seven and Citizens of Humanity and often have codes. they have free shipping.
jcrew, abercrombie, etc. have cheap shipping to Canada BUT they ship via UPS/Fex/DHL/etc. which all charge "handling fees".