Set shipping prices in the auction
The easiest way to avoid any dispute within Ebay is to set your shipping prices clearly within the auction itself. Your description can include details of costs within Ontario, within Canada, with N. America, etc. Add your costs (listing, Paypal, etc.) to the actual postage and insurance and come up with a fixed shipping and handling charge. Any bidders that win the auction swill have no recourse but to pay your advertised shipping costs.
You may alienate some bidders that feel your costs are too high, but it is far better than having to argue with someone that has already won the auction.