Well the important thing is that the building has at least one scaling value.
So if mail radius is fixed, perhaps Delivery Time is what gets better as a result of investment (and again: requires a certain amount of constant investment to maintain.)
What I'm suggesting are basically area of effect benefits. The idea that individually sure I could run that post office and get the exact same benefit as a village. But in a village only one person runs it and all reap the benefits. And if many such AOE benefits stack together, the village becomes much more than the sum of its parts. Everyone in the village becomes that much more efficient.
The continued upkeep part of the suggestion is also crucial to the mechanic I'm suggesting. If you play 40 mins/day and keeping the Mail building running takes 10 mins, that's doable. If there's 3 other building types at 10 mins/day it's still doable but eats up all 40 mins.
But since these are village-wide effects, in a 4-man village each player only has to spend 10 mins/day on the stuff for everyone to get the benefit of all 4 buildings.
(Buildings wouldn't be the only way for these effects to exist. Perhaps the chief automatically provides an Inspiration Bonus to his village when he works hard each day (earns a certain number of LP). So it's not a building, but it is the same game mechanic at work.)