Solarpunk

Fair enough. I did not see it as a prompt.

I guess that problem becomes really much simpler once you have a fleet of autonomous electric vehicles and decarbonized grid. From heavy trucks down to individual light-weight drones dropping things on rooftops/balconies/urban dropsites I think the problem will pretty much solve itself, whatever economic system is behind.

Yes, you will likely need some central nexuses depending on how your transportation network is organized.

a solar punk society would still have to figure out how to cooperate with postal systems of different continents / societies. Imagine having a solar punk society trying to make a deal with current USA on how their postal systems should cooperate.

Postal services were one of the first thing to organize internationally. The UPU was founded in the 19th century. The problem in itself is simple:

1. Agree on how to state which country to send it to.
2. Write an address in a way that locals understand it
3. Pay the local post the fee they consider fair to distribute it.

I consider this problem solved and I don't see why we should assume it becomes more complicated than that? If you live in a place with free deliveries, then you may still have to pay a fee to deliver to another country. And from another country you will at least pay the fee to get your stuff to the border.