USPS shipping rates to Canada. This is the official USPS site. Again, seller was correct.
http://ircalc.usps.gov/MailServices.aspx?country=10054&m=6&p=2&o=0
Do you know that for a fact?
The seller clearly stated in his listing that shipping was $11 to the United States. No international shipping was offered. Seems to me it was the buyer who tried to change the terms, not the seller.
Like I said, the concept is the same as a rubber stamp. The rubber stamp takes the inked design from one place and applies it to another place, just like the printing pad takes the inked design and transfers it to another place.