I don't know your situation of course, but back many years where I lived in Texas, there was some kind of mail box tax, and somehow the rural boxes were maintained by the municipality, on very similar posts/bases..
But that was years ago, and in east Texas..
Here, the box height is 41".. and they still all too often get covered in snow...but of course, the height is configured to the delivery vehicle..
A couple suggestions.. Since you have clusters of boxes, you could use a couple 4"X4" or 6'X6' posts, perhaps 10 or 12 feet apart.. and string a couple 2"X6' boards between the posts..which would be much easier & thriftier, than a single post for each box.
Another suggestion.. Since apparently there are mainly seniors there, do you have a Boy scout troop in the area? Some scout or scouts seeking their eagle badges, could do the needed work as an eagle project...perhaos they only need the suggestion.
Barring that, is there an American legion post nearby? Perhaps they would consider it a "fun project" to do the work.
How about a nearby Home depot...great chance for a cheap and easy project..with lots of free advertising !
Seek out the manager..talk to him/her..you will never know, if you don't try..just impress him/her with the public relations value of such a simple project !