Free shipping is often only domestic. It is unusual for sellers to offer Free Shipping outside their own borders.
Some (stupid) sellers will put ridiculously high shipping for international destinations to discourage foreign buyers.
Did you only see the Free Shipping icon? Did you check the Shipping and Handling tab to confirm the cost of shipping to Canada was actually free?
Was the item sold through the Global Shipping Program? It is possible that shipping is free, but your item attracted duty and sales tax. It is not unusual for a very valuable item to be offered with no additional shipping. But it is still subject to Canadian import fees.