Taking it further, what if the flying box could rendezvous with a moving vehicle? Preregistered vehicles could then pick up / drop off parcels without deviating in any way from their already planned journey.
That sort of tech could even be useful for existing courier companies, as it would make pick up and drop off of small items very efficient. The truck would just drive a predetermined route, that passes within 1 km of each point of a city, without stopping.
It could make an interesting partnership with the local bus company. Each bus would have a roof mounted drone docking station. The drone would fly from its start point to rendezvous with the nearest bus, then hop from bus to bus until it is near its destination, and fly the last mile.