The politifact article cites its sources, including https://about.usps.com/news/statements/080117.htm which says "By law our competitive package products, including those that we deliver for Amazon, must cover their costs."

Whats your source that they charge amazon less than it costs them to deliver?

Feel free to link the sources from your Facebook feed claiming otherwise. You're clearly the type to thoroughly vet their news source. I have no doubt it'll be ironclad.