The safe idea is interesting, though.
It's a somewhat different experience to a human knocking on your door, but I'm sure it could be made to work.
Automated delivery vehicles may be able to do better in terms of offering a choice of delivery time, and sending accurate estimates of arrival time. There's considerable scope for doing a better job of delivering to households where there is usually nobody home during "working hours".
What you're essentially describing is a motorised set of po boxes. Which would work brilliantly. For pizza delivery it's a motorised warmer oven with multiple components, probably looking a lot like those little motorised droids that are always zipping around the ankles of actors in Star Wars films.