Since people can put items into their cart before logging in (or even before registering) it would require the Mind Reading mod, available in the Free Software Add Ons. (Just kidding. I haven't made it publically available yet.)

So you would have to wait until the checkout process starts, then use a conditional bit of code to check the country code (if not CA) and check the product ID #s and if there's a mis-match, pop up a warning. What you do at that point is up to you... cancel the whole order? auto-remove that one item form the cart? or something else?

Rob