" cost of delivery from solar-power-recharged inexpensive drones is going to go to near zero"
Solar electricity is much more expensive than electricity from other sources, more importantly, 'fuel' a very minor issue in the 'total cost of delivery'.
The 'cost' will be in so many other things, first off - having inventory within a 'drone flight' away from people.
I actually don't believe that drones will ever fly stuff to our door.
It will always be more expensive to delivery stuff 'by flying drone' - and 'very quickly'. See: prices for airmail vs. snail mail.
So what do consumers really need that they can't drive to the store for? Toothpaste? Books? Toilet paper? No. That won't be cost effective.
If someone 'really needs something now' - the 'cost effective' way to do it is to walk or drive down the street to the store.
Delf-driving trucks, with 'robot arms' to nicely 'drop stuff off' for the rest.