The problem is many Canadian buyers THINK that intraCanada shipping should be cheaper than say US-Canada Shipping.
There are a lot of CDN buyers who just have NO CLUE as to how much it costs to ship here at home.
I've been raked over the coals by many such ignorant people and there's not too much you can do about it.
You did the right thing to block them upfront. The worst is when they bid and then complain after they win and tank your stars - after you LOST money to ship their item out (flat rate lower than actual postage).
Sometimes, when dealing with these people I try and educate them. I will send them to the calculate a rate area at the CP site armed with the dimensions and weight and my postal code so they can see for themselves.
It placates some but the others, who are looking for a fight, will say the dimensions must be wrong.
Sigh.

I am the guardian of my children and their future. It is for this reason that I scrapbook.