Wow, looks like they had one supplier of trucks but would buy airplanes from whoever was selling.
My guess would be that the trucks are postal service vehicles and the airplanes are subcontracted out to who ever owns one and wants a job flying mail.
Those are ambulances removing all the dead letter mail. :-)
Who wants to take a crack at ID'ing the airplanes? In the nearest row I have a Stinson Detroiter, Travel Air, Ryan Brougham, Douglas M-2(?), then it gets kind of harder. Maybe a Boeing Model 40 behind that? Along the back are more Douglas biplanes, and Boeing(?)s, a monoplane I can't quite place, and right under the NAT on the side of the hangar is a Curtiss Carrier Pidgeon, a mail plane only a mother could love.
There a movie the Keauhou reeves called "the Aviator" don't confuse with that terrible headache Hughson movie
About airmail pilot
It was Christopher Reeve not Keanu Reeves that stared in the 1985 version of The Aviator. It was no block buster either.