Re: sounded logical but.....
"However the problem is real; there is neither the generating capacity nor the infrastructure cabling right down the users' premises (including substations) to cope with a widespread migration to electric "
In most places, there is the capability. Just not during mid-day but rather in the middle of the night when usage is low. Since the mail isn't being delivered at night, the vans are just sitting around.
A postal van is going to be a purpose built vehicle and isn't going to have 27 cups holders, widgets and gizmos all over the place and a double insulated chassis for perfect silence. It will be built light and get much better "mileage" than a consumer EV with vibromatic deluxe seats all around. It's also not likely to come with a battery that gives it hundreds of miles of range. Optimizing the battery capacity to a few range brackets will add to the efficiency since a van that only goes 50 miles on a charge will be carrying around far less battery weight than a Tesla that may go 400 miles before needing a charge. A rural van may have a bigger battery to make a longer daily run or there might be a charging station along the route so a van can recharge while the postman has lunch. Chances are that there will be a power substation that can be easily fitted with chargers for power company vehicles and available by agreement to other government agencies.