I'm not sure what caused Trump to tweet on this issue so much recently. I think it is because many people said his initial tweet was wrong about the Post Office losing money.
The Post Office loses money every year. If you take their annual loss divided by the number of items they deliver per year, you can come up with an amount lost per package, which appears to be how Trump determined that the Post Office is long money on Amazon packages. Another way to look at it is at the margin. If the Post Office delivered enough items per year, it would not lose money. Every additional item delivered by the Post Office reduces the amount lost per item. How would someone with a lot of business experience look at it?
A deeper dive.. https://www.vox.com/2018/3/30/171768...on-post-office