Everyone is missing the point. It's not the form of the conveyor that matters it's the loading and unloading system. You need a set of standardised parcel sized containers first. Then you can design robotic handling equipment that can load and unload them easily. You can take the containers and feed them into your postal system and gradually automate more parts of the system reliably once you have standard reusable containers, bonus points if the containers are collapsible like some crates you get for fresh produce. You can imagine them clicking magnetically into totems on the street where they could be collected robotically more easily. Or in high traffic locations you might feed them hole in the wall that takes them into an underground system of some kind that transports them to a rail or truck depot. You can also imagine that parcels services might be reintroduced on local trains because you don't need an extra person to load and unload the containers at stations if this can be done robotically. There are endless possibilities for varying degrees of automation, but the key thing is that there has to be a standard interface for picking up a container of a known size. The parcel is the interface; the physical specifications of the standard containers are analogous to an API.
The technology already exists for this in airports[0][1]; when you check in a bag in a big modern airport after the gate agent sticks the sticker on the handle it won't be touched again by a human until it gets chucked into the aircraft hold. Your bag goes through the curtain and it is dropped into a standard bucket which is conveyed around under the concourse on a rollercoaster like automated rail system to screening then either to an automated vehicle for transfer between terminal buildings or to an automated storage system for people who have checked in too early or have a long transfer, finally to the stand/ramp for loading into the aircraft. Big international airports like Amsterdam Schipol, Paris Charles de Gaul, Madrid Barrajas and Heathrow all have systems like this.
The technology already exists for this in airports[0][1]; when you check in a bag in a big modern airport after the gate agent sticks the sticker on the handle it won't be touched again by a human until it gets chucked into the aircraft hold. Your bag goes through the curtain and it is dropped into a standard bucket which is conveyed around under the concourse on a rollercoaster like automated rail system to screening then either to an automated vehicle for transfer between terminal buildings or to an automated storage system for people who have checked in too early or have a long transfer, finally to the stand/ramp for loading into the aircraft. Big international airports like Amsterdam Schipol, Paris Charles de Gaul, Madrid Barrajas and Heathrow all have systems like this.
[0] https://www.youtube.com/watch?v=LVesQ07GrRY&list=PLWwq_41dNV... edit: [1] https://www.youtube.com/watch?v=cac411oBqSE&t=57s