The USPS does have a passport desk in one PO in Boise.
The PO takes the application, gives you a quick look and the passes it in to the state department to make the final decision and issue the passport. When I got my first one I just mailed it in but now they want an actual government employee to match you with the picture you gave and to see you sign the forms. All the PO is doing is acting like a super-notary for the forms.