USPS has 231,000 vehicles. If 200,000 of them became electric, that's 3x more than GM has sold in Chevy Bolts since 2017, and could be considered high volume.
Being a high volume vehicle, it would make sense they were purpose-built for competing with UPS and Fedex. They'd have efficient stop/in/out/go driver ergonomics, totally keyless with automatic parking brake, large LCD panel screen with tactical delivery information, easy package storage/retrieval/scanning, etc.
The average postal route is 25 miles with 500-700 stops. The vehicle should easily get >= 2.5 mi/kWh, so the entire battery pack could be just 30 kWh ($4000). All vehicles plugged in at the end of the day with computerized round-robin EVSE sharing. Then natural gas vehicles for anything longer range.
Unfortunately the USPS is a bureaucratic dinosaur with little regards to efficiency or customer service, so forget the whole thing. Maybe in 2040.