But... how does the truck have the inventory to fulfill whatever is ordered, within 30 minutes? You don't know in advance what "the day's deliveries" are going to be, or where they will be, because the drone deliveries are on-demand.
Amazon could get a product to a local centre, then instead of sending a van out driving all day going to one customer after the other, they'll hit a few key locations near multiple customers and launch drones out to serve them at the same time.
That said, you could probably extend the range of the 30-minute service with a strategy similar to this. Just bundle orders for 10 minutes, send vehicles 10 minutes out and then the drones fly 5-10 minutes to deliver the package.
Additionally, you could probably keep delivery trucks stocked with some of the top items as well and surprise/delight the customer with "Hey, we just so happen to have 30 minute delivery available now if you want it." I'm no SCM expert, though, so that could very well be a waste of more-valuable space.
The actual mechanisms are actually super straightforward -- basically if a widget X sells Y amounts per day with Z variance in an area, get the low, low end of that to that warehouse/area so you only have to do last-mile delivery.
Then do it with operations research (well, linear algebra) on scale. :)