The procedure is exactly the same, whether you do it at the post office or on your own. The only difference is that you can wait for an appointment there to have the forms checked for you. they approve and process nothing unless you are required to appear in person, then the post office swears to your appearance.
Just like 29 palms said--you complete the forms, you submit the forms for postage at the window and the application goes to the same place as everyone else's, unless you pay the expedite fee. If you want to bet your passport comes back in less than 2 wks, go ahead. I prefer better odds on the lottery!! Remember--this is a gov agency!!