I've never seen a full analysis. Twenty years ago I received and sent a lot more first class mail but got fewer packages. Are the packages killing the PO? I know most of the big shippers have zero loyalty to the shippers. A small store might use USPS or FedEx for everything, but the big ones like Amazon will have their computers figure out for each package which shipper is cheaper and faster or for Amazon just deliver it themselves.
“Where is the actual biggest cost for the Post Office?”
Once I think I heard that it’s the extravagant pension program. Not sure, though.