If the tires are not made in Nafta Zone - then you'll get hit with 6% duty, then HST

Did you look at DiscountTireDirect - they usually have free shipping to lower-48.
What about the $5/tire fee at UPS? I assume that's where you have them shipped?
If you got the sent to Latham Tire (
http://www.lathamauto.com/) in Oggy they might not charge anything to hold them - they are a TireRack & DTD approved installer - I had my Yokohamas for my old Corolla sent there last May - drove down on old tires on good rims, had the new Yokos mounted on the rims and continued to Carlisle

You have to watch that though and be honest at the border - they'll ask if you had anything work or repairs done to your car coming back across.