I used an expeditor for a first passport a few years back and the way it worked was:
I processed all the paperwork at a post office and put it in an envelope sealed by the postal employee;
I sent the envelope by Fedex overnight early delivery to the expeditor;
the expeditor processed a same day passport (they have special access);
the expeditor returned the passport by Fedex overnight;
I received the passport the next day.
This saves you a trip to the passport office and any problems getting an appointment as long as you have three days to do it. I think you don't need to involve the post office for a renewal. It is certainly a good way to do it if you don't live close to a passport office.