Free Shipping from Shenzhen to Toronto: How?

Just have to realize that everything you ever buy from China is final sale due to the cost of shipping it back.

The foot pad on my Macbook Pro Retina fell off the other day. The Apple Store didn’t have a replacement part but an eBay seller in Shenzhen, China did. For $3 with free shipping. The cheapest international shipping option with Canada Post is $5.35 for up to 100g (without tracking). How can a store in China sell me anything for $3, including shipping?