Look, I don't know where this idea came from that electricity is this hard problem. It's not. As it is now, many of these LLVs have to be fueled sometimes multiple times per week. Simply plugging into an external outlet not only is cheap but will actually save time.
People who do not have electric cars keep looking for an inventing some kind of "gotcha," but they really are this easy. You literally can just use a normal external outlet to charge them, and even if you want faster charging (totally unnecessary and probably undesirable for this application), it costs maybe $1200 to get an RV hookup (equivalent to L2). I charge from an old dryer outlet, although sometimes use an exterior 120V. It really is that easy. In fact, I don't know if big USPS locations have refueling depots like bus depots do, but if they went electric, they'd be unnecessary and could be removed.