why not just use the delivery (UPS/Fedex/etc) trucks as a base station for the drones?

Pull in to a neighborhood of, say, 500 residents; Say you have about 100 packages to deliver... Set up 5 drones to deliver packages. Even if it takes 60 seconds on average for each drone to deliver and return, it probably is still much faster than the truck driver having to drive into every street, stop, grab the right package, drop it off, and go back to the truck and repeat.