Earlier quoted context omitted.
At most post offices the trucks are parked outside without electrical outlets nearby. Sure those can be built but it will take a while. In many cases the circuit supplying the facility will have to be upgraded.
Any parking lot with lamp posts has electricity. Takes way less time to install an exterior outlet than it does to build a vehicle.
Are you going to run extension cords for the 150+ feet to each outlying vehicle? No?
Then you have to tear up asphalt to lay down new lines. That's not cheap, especially with a USPS that's so strapped for money right now (for political reasons).