I live in Vancouver and buy stuff from the US all the time.
As for tires: If they are made in the US there is no duty. There is, however, GST and PST assesed on the Canadian dollar value of your purchase.
Now, there is also the issue of brokerage.
I have a US address in Wahington State that I ship everything to. Then I go pick it up and drive the stuff across the border myself (I act as my own customs broker).
If you use another customs broker (UPS, FEDEX, USPS) then charges may apply for their services. Fedex has no charge, UPS has a ridiculous charge of about $30 although I think it fluctutates a little with the value of the goods you import. The USPS has a $5 brokerage charge for Canada Post to collect the taxes due.
So, if you shipped the tires to a nearby border town (maybe detroit is closest for you) you can count on adding PST and GST to the CAD value of the tires. If you have them shipped to canada I think you will find it pretty costly (although you might still save money.)
If the tires are _not_ made in the US then you will have to pay duty as well as PST and GST. IIRC duty is either 4.2% or 6.2%.
see ya,
