Canada post is the cheapest...

It was probably people who listed their item as being in the US, but who were actually shipping from China or India. Shipping seems to be incredibly cheap from those two countries.

Or it may have been sellers who listed the shipping for $8, but were willing to take a loss on the shipping to sell the product.

Either way, you're not going to find much cheaper than that for shipping to Canada. If you purchase the shipping label through eBay, you'll probably save some money from those estimates, but I don't see it going much lower than $15. :/

Canadian here. I order stuff from the US all the time. Some times I randomly get hit with huge import fees (like when I'm ordering 3D printed models from shapeways.com) and sometimes I don't get hit with fees at all (like when I order from Amazon). Other people I have talked to have had the same experiences.

If you are going to be moving a lot of product into Canada, I would suggest speaking to an import agent. Just my 2 cents