Those companies performed a physical service. I fulfill my own orders and don’t use any Amazon services. Like I said, they host a nearly static e-commerce page, and the typical cost of that is about 4%.
I have no idea what it would cost to use every possible Amazon seller service, but it would probably get into the range of traditional retail. Also, Amazon automates race-to-the-bottom online price competition, so they’ll be getting 13% of the net sales even if prices were automatically cut to the point where the seller is taking a loss.