In 1783 the Montgolfier brothers flew through the skies for the first time ever in a hot air balloon. It wasn't even sure if humans could survive being in the air so the King wouldn't let them send humans aloft until test animals survived first.
Later that same year, the hydrogen balloon was invented and that one was flown to an altitude of 3 km high.
And yes, the idea that you could write in Edinburgh and have it delivered to Berlin had already been invented much further back than after 1823. The Roman Empire had the Cursus Publicus which handled mail delivery between the Provinces and Italy.
Do you mean someone of a high enough rank could have official communications and military dispatches delivered, or do you mean any person could have any letter delivered to any home? I think tialaramex was writing more about universal civilian letter delivery, and particularly about universal (well, global or near-global) addressing, rather than letter delivery between officials within an empire.
There would only be a need for that with ubiquitous literacy. The Roman system may well have been close enough to universally accessible and offer delivery throughout their territory for Roman citizens who were literate.
I don't think it realistically did - it included a system of authorisation that implied a demand that was intentionally denied, however there are multiple other old postal systems which definitely did (e.g. the Imperial Mail of the Holy Roman Empire [1] being one of the significant ones, but that itself built on older systems). A more defensible claim would be the strictly limited claim that the idea of attempting to create a global postal system with global addressability is a new idea. But even long before the UPU, a lot of countries had treaties to allow for forwarding of post, so I think it's unlikely that this idea was new as well, just previously impractical.
If something has been invented, the idea "the exact same thing but cheaper" has definitely already entered peoples' minds. They may not know a good method to do it, but they can imagine a world in which it exists.
Later that same year, the hydrogen balloon was invented and that one was flown to an altitude of 3 km high.
And yes, the idea that you could write in Edinburgh and have it delivered to Berlin had already been invented much further back than after 1823. The Roman Empire had the Cursus Publicus which handled mail delivery between the Provinces and Italy.