I would ask you to define "central city". Cental based on location? Businesses? Population? On the counterside, as people drive in 40 miles to go to the PO, mail carriers drive out 40 miles.

On another note, the USPS is becoming obsolete. Private companies have developed a better more sustainable business model and everything is going paperless anyway. I would rather have tax payer dollars go to something else. Any money that is spent on things other than sophisticated logistics equipment would be a waste like moving a post office to the Federal Building.