You have to understand that a lot of sellers on eBay are casual sellers who may not understand how to get the best possible shipping rates. If someone tries to ship a Hot Wheels car using Canada Post's counter rates, the minimum it would cost would be close to $20 ($16.99 in a flat rate box plus tax).
eBay has a service for packages under 200g where they can be shipped for about $8 via Canada Post, along with whatever handling fees the seller feels is appropriate. Keep in mind that sellers have to pay for supplies. They also only get about 80-85 percent of what you pay for shipping because eBay charges fees. So if the shipping cost is $8 out of pocket, a seller might tack on a $2 handling charge so that they don't lose money shipping you your item.
There are a few alternative shipping carriers that can ship small parcels for $6-$10, but they aren't accessible to the average person. You really need to be a full time business that is shipping multiple items daily for the time and effort of dealing with an alternate carrier to make sense because they usually only have drop spots in select locations that take time to get to.
Some sellers do ship items for cheap using lettermail. This is limited to very small items that are less than 2CM thick. Additionally, sellers aren't covered if the item is reported lost by the buyer. So not every seller is going to be willing to use lettermail. Especially not casual sellers.
I think it goes both ways in that some sellers do overcharge for shipping, but also some buyers don't completely understand the reality of what shipping costs. Large firms like Amazon have skewed the expectations of what shipping costs. The reality is, small businesses on platforms like eBay and Shopify aren't in a position to sell things as loss leaders like Amazon might be with an item both fulfilled and sold by Amazon.
The TL;DR is that it costs $20 for an average casual seller to ship you something tracked with Canada Post. Even something very small. $10 if they use eBay labels and it is below 200g, but not every casual seller knows about that service.
