Amazon takes 50% cut from small businesses
bloomberg.com
bloomberg.com
The hard lesson here was that you build your business on other people's platform at great peril, and it is almost never worth it over a long enough timeline.
I don't disagree with this, but for customers, this is a bonus: I don't have to give random small business my e-mail or any information. I, as the customer, tend to trust Amazon more than Small Co. with any personal and billing information.
I hate that when I buy things using Shopify's payment platform, there's an auto checkbox that says "Allow store to contact me with deals and offers" that I have to opt out of.
This is a naive view of how the economics work here and the power these platforms hold. The point of the OP is that sellers eat this cost, for the most part. Some of this is passed to the customer, but not a significant amount. The prices on Amazon are as low or lower than anywhere else (this is part of the problem).