As a Canadian, I could never understand why a U.S. based seller of cards (baseball, hockey, etc.) would not ship to Canada. There are 30 million of us hosers up here, you know.
Best thing to do, if possible, is mail inexpensive cards regular mail, charging $2.00-$4.00. Anything deemed expensive (to me, over $100.00) should be sent with tracking, costing $10.00 and up. If a Canadian buyer is not agreeable to paying for this, have him/her move on.
It's the idiot buyers like the original one in this thread that cause the trouble, but they are the very small minority.
Also, it works both ways. Many U.S buyers pull this stunt on Canadian sellers, since they know that tracking parcels from Canada can be expensive, and sellers will often not bother. Without delivery confirmation, the seller is out of luck