Air mail already exists. Some people pay the premium for faster delivery. I don't think price benefits from going electric, or robotic, would be significant enough to change the existing market incentives. The last-mile door to door mail delivery by road is quite efficient, with several deliveries daily by the private companies (UPS, DHL etc). Of course the situation is less good in less developed/urbanized/rich areas, but that is due to less demand, not so much because DHL couldn't provide the same service there if demand existed.

Ditto for personal commutes to work. Robotic planes may be faster overall - although I would like to see evidence; someone who knows how existing aircraft are routed should comment on the plausibility of one-small-airplane-per-person doing a daily commute in a densely populated area. But since flight costs much more per distance traveled, people won't pay the premium. Also, faster and cheaper ground travel (e.g. trains or metro vs. cars) has its place.