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?
There key reason why it's so cheap: a UN treaty called the Universal Postal Union.
The Universal Postal Union (UPU) started in 1874 as a way to make inter-country mail more convenient. Before the UPU, senders would need to affix stamps for all of the countries along the way between the source and destination.
An amendment to the UPU treaty in 1969 introduced a terminal dues system that provides for cheaper shipping from developing countries to developed ones. The system is rather complicated but this whitepaper by the US Postal Service explains how it works and why it results in cheap shipping to the US (and the same principles apply to Canada).
The Washington Post explains more here: https://www.washingtonpost.com/news/storyline/wp/2014/09/12/the-postal-service-is-losing-millions-a-year-to-help-you-buy-cheap-stuff-from-china/
The end effect of the UPU is that Chinese sellers have a big advantage over domestic sellers when it comes to low value goods sold online. It's an outcome that no one would have dreamed of in 1874 but it's certainly achieving the objective of the 1969 amendments by helping China develop.
In the United States there is also a special deal between the Chinese postal service, the US postal service and eBay: http://about.usps.com/news/national-releases/2010/pr10_058.htm. Perhaps there is a similar deal with Canada Post that isn't public.