if US Postal Service was willing to buy foreign vehicles,

It's not. Offshoring critical infrastructure is always a bad idea.

they'd have to deal with EPA and NHTSA rules

Possibly not. They don't even have license plates, as the Postal Service outranks state governments.

The NGDV's gross vehicle weight rating is 8,501lbs so that it is classified by the EPA as a heavy-duty truck. If the rating were a pound less, it would be classified as a light-duty truck and have much stricter emissions standards.