There was a time when these were built by Jeep/AMC motors. People used to buy these used at auction and drive them after they were retired. And electric delivery vehicles are already built by companies like Rivian and Ford, though I don’t know what the cost difference is.
I would think building a “bespoke” vehicle for USPS would result in more expensive service parts. But I don’t know what kind of service contracts are included with these.