Go back a step.
First. Are you sure that the Free Shipping also applied to Canada? It is not uncommon for the Free Shipping icon to appear, especially if you are looking at the dotCOM site, when only Domestic (US) shipping is offered.
Go to the listing and check under the Shipping and Handling tab.
He can't change that information once there is a bid so if you do see a shipping charge you do owe it.
If there is no Free Shipping offered to Canada, you have two choices.
You can contact the seller, apologize, explain your error and ask for a Mutual Cancellation. This frees both of you from the transaction and you can leave feedback. If he allows this, and he is not required to, accept the offered Cancellation promptly and leave positive feedback. Remember, the error was yours.
Or you can not pay. The seller will then open an Unpaid Item Dispute and if you still don't pay, you will get a Strike that will make it more difficult to bid on eBay. You will not be able to leave feedback.
