It's likely that the energy cost of drone delivery is a small fraction of the cost a human delivery driver incurs walking to a stoop, particularly if one considers the not insignificant injury risk.

flight is appealing over ground based drones as one delivery truck or base station could concurrently service multiple blocks with lower risk of someone abducting or damaging the drone. Although in all honesty I'm not sure if weight, range, carrying capacity, or speed would work out in a compelling form factor.