Collect what you want and hopefully enjoy doing so.
My point is that there is simplicity in one organization, the UPU, handling the mailing rules (In peace and in war), otherwise just for the 192 UPU countries, there would be about 18528 individual mailing contracts needed. Likewise membership in the UPU is not difficult.
Malta (SMOM) has by your count 50 local post routes. It is, in no way part of the integrated worldwide mail service conducted by the UPU since 1874. The population (not membership) of SMOM is three citizens, far lower than the 28 in Lundy which requires a local post to connect it mail to the world.
The stamps of SMOM fall into the category definition of Cinderellas, as they are not issued by a governmental postal administration. Even Lundy is a local post yet, since 1929 some 350 Lundy stamps have been issued, many of them now very rare and highly collectible. Some 40,000 pieces of mail are sent from the island every year and Lundy is the oldest private postal service operating in the world today. Like SMOM the Lundy stamps are not valid throughout the world.
You appear to be defining anything that is not "official UPU" mail carriage as a local post.
Spot on as that is how such is defined, not by me but by organized philately, as noted above. As to the 50 contracts you keep mentioning for the three citizens, they are with countries in which SMOM provides charitable work. Almost sounds like a pay to play arrangement in an attempt to gain more political acceptance. Perhaps someday it will be elevated from UN Observer status (since 1994) to full membership if circumstances arise to validate such change.
Edit: I need to come back and correct the number of individual mail contracts needed if the UPU did not supplant the need.
Since I am now here, I will address your quote from below
I'm just saying that your initial statement is not entirely accurate.
Yes it is as applied to countries. SMOM is not a country, it is an organization, a collective, a religious association, a government in exile, what ever you wish to call it but as it has no territory, it is not a country and does not have borders outside of which to send mail. I gave cross border examples of local posts as a qualification of the UPU statement, and they are local post. Once outside of the narrow, small areas the local stamps are accepted by private agreement, they have no public validity (just private local recognition) in official governmental postal services anywhere else in the world.
By the way I used the term stamps meaning what is generally accepted as a Cinderella. SMOM also has issued aerogrammes and imprinted (postal stationery) I have no idea what to call them as they don't meet the Cinderella definition of "stamp" like.