What happens when a Pigeon goes down? And what happens when a Smart Case is dropped?
Most if not all flying Pigeons will have a standard Top Plate connector on their top surfaces so they should be able to be picked up and carried by any other Pigeon with a heavy enough payload rating. This means that if they lose power or break down they could be treated like a large Package and simply be delivered to a workshop for fixing. Sometimes Pigeon will land upside down or in difficult to reach places. This is where the Hawk comes in.
The Hawk is a Semi Autonomous Multicopter or ground based platform that is deployed to search for and retrieve fallen Pigeons and dropped Packages. It will need to be much more powerful and capable than a Pigeon and during the retrieval stage it would be controlled by a human operator with a First Person Video feed. The grabber or sky hook will also have to be quite dexterous to get at awkward stuck packages.
It would be role of specialist PPP Service Providers to develop their own Hawk platforms but here's a video to show what could be possible:
As a last resort, if a Hawk cannot retrieve a Package or Pigeon, a human can always be dispatched to do what humans do best.
Using Traditional Carriers as a Backup System
Although we want the system to be robust there will always be failures. Smart Packages which are dropped and found in areas not reachable by Hawks could simply be picked up by members of the public and dropped into the nearest post box. Agreements between traditional mail carriers and PPP Service Providers could be established to provide this back up service at a prearranged price so there is no price gouging.
Bounty or Honor System?
What incentives do members of the public have to forward dropped Packages? A bounty system of rewarding people for retrieving package could be established though this could have unintended consequences of people forcing the network to break just to get paid to fix it again.
A better system would be one based on mutual understanding that market based systems work most efficiently in low corruption environments.