Where are they going to charge them once a post office has a few? I thought I read somewhere that charging infrastructure is fine for a household but a fleet of 10+ EVs will need significantly more juice (technical term) and hardware to mitigate significant safety concerns.
Some searching brings up 24 miles a day as the average for a postal route. Even if you round up to 50 miles a day per vehicles, a 400A service will do more than 50 vehicles at night. It's not ideal for trucks between facilities, sure. But for neighborhood last mile delivery you wouldn't have that many vehicles per facility, or distance traveled, making it pretty ideal to start rolling out electric vehicles there.