I'm not the original commenter, but here's two sources [1] [2] for the 50% number, with [1] saying this represents 5% of US retail sales. It's 50% of US e-commerce.

[1] https://techcrunch.com/2018/07/13/amazons-share-of-the-us-e-... [2] https://www.cnbc.com/2018/07/12/amazon-to-take-almost-50-per...