I had this message when I tried to buy NGK Iridium plugs from Summit because they were much cheaper than buying them HERE. You are in Canada. I suspect NGK has a deal/policy with Summit to only sell within the USA.
Your suspicions are correct, I contacted Summit today and they told me that there is a "no ship outside the US" policy in effect.
"I am sorry we are not able to sell NGK spark plugs outside of the USA.
This is due to NGK contract agreement."
That I just do not understand why.