I have purchased from the Tire Rack in the past. I did, however, have the tires shipped to a friend's place in Rochester, NY, where I picked them up and transported them back into Canada. Since the tires I bought (Goodrich KDW) were manufactured in the US, there was no duty charged. PST and GST was applied at the border. I suppose I could have avoided PST/GST if I had mounted the tires in the US, but I would have been bringing my winter tires back into Canada, which may have raised a bit of suspicion.
If the wheels/tires you are interested in are made outside of NAFTA countries, there will be some duty to pay. If you order from Canada, expect to pay PST/GST, possibly some duty, and a brokerage fee. The Tire Rack processed and delivered my order in a very expedient manner, and at a great price.