By the time SDC are here drones should be prevalent, which will take care of the last mile.
What am I missing?
Although I suspect a compressed air tank with enough capacity would be way too heavy for a drone to carry.
The power to weight ratio is the key issue. Also -- noise could be an issue too -- want to have quiet deliveries.
I'm thinking parcel cannon + small airbag bumpers and maybe a little parachute to slow decent at exactly the right time :)