Very hit and miss. Due to fluctuations with the Canadian dollar and uncertainties over customs duties and imports, I find a lot of these places either won't ship to Canada or will slap some sort of "import processing fee" on the orders.
That's not my experience at all. I've placed dozens of orders with WhoNA over the last eight years, and at least a couple with Mike's Comics (their refusal to do credit card transactions online is why I never used them regularly). The shipping is a bit expensive, but that doesn't appear to be a markup issue.