It sounds to me as though the "International Priority" shipping you're referring to is shipping through the Global Shipping Program. There should be a banner near the top of listing pages using this shipping method that states that the Global Shipping Program is being used and a link to further information.
In a nutshell, this shipping method involves the seller shipping to a warehouse in Kentucky which then does the necessary customs paperwork and then ships the item in a container-load of freight to Canada, where it clears customs as part of the freight shipment before getting separated from the container and forwarded to you by another shipper (likely Canada Post).
There are scads of threads on the Global Shipping Program on this board and others. It's generally acknowledged that it's not a good shipping method for small and/or modestly-priced items, but the sellers that have adopted its use either don't know they've registered for it or else they haven't done much international shipping prior to using the GSP and have no idea how much it can cost buyers using it.
