USPS is amazing, truly one of the wonders of the world.
They have a pretty tough mandate though, which is to deliver 6 days a week to every mailbox in every little rural ghost town in America, for a flat rate across the whole system. Their volume has been dropping for years as people transfer routine communications online, and they have to keep funding pension obligations for retired workers from the years when volumes were higher, and their workers are all treated amazingly, much better than their competitors. They often end up losing the price competition on higher-margin packages to private shipping companies, but those companies are happy to use USPS for last-mile delivery whenever they need to, and so don’t need nearly as complete a network.
Not to mention, congressional Republicans keep trying to defund it.