Logo of the MPHS
The Military Postal History Society (a non-profit corporation) was founded in 1937 as the War Cover Club, American Philatelic Society Unit #19. It promotes the study of the postal aspects of all wars and military actions of all nations.
Dues for new members are described on the Military Postal History Society Membership web page.
Membership dues amounts depend on the time of year you are joining, along with the format in which you want to receive the award-winning Bulletin of the Society.
The Military Postal History Society Web Site is well worth a visit.
Note that the publications of the Military Postal History Society have been very helpful in the preparation of my book Domestic United States Military Facilities of the First World War (1917-1919).
These are excellent reference documents, as is the Bulletin of the Society. Back issues of the Bulletin (to 2017) are freely available on the website.