|Amazon wanted to lower delivery costs and speed delivery time, and deliver on weekends.|
FedEx and UPS rolled out expanded Dimensional Weight, which was tougher for smaller companies to measure and bill, and therefore charge their customers. Bezos obviously, got USPS to open Sunday deliveries, which FedEx used to charge $10 for (for each delivery). USPS has the infrastructure to do weekend deliveries, and incremental business from Amazon is profitable.
If Amazon goes more with FedEx, UPS, DHL, it would be worse for the USPS.
The USPS problem is mostly legacy healthcare and retirement costs.
If Trump continues with his grudge, Bezos may offer to privatize the USPS.