Those are very wrong assumptions. If a serious drone delivery program (or multiple competing ones) starts, airspace will suddenly become congested and you'll have to do heavy air traffic coordination to avoid mid-air collisions. And when you comapre with cars, you need to compare one delivery car with a hundred drones. Suddenly you see that drones are not efficient at all, which is kind of obvious, as it takes a lot of energy to keep stuff in the air with active propulsion.