The story
Postal services in Monaco are provided by France pursuant to the Treaty of Péronne in 1641. This relationship has been interrupted twice. The first interruption occurred during the French Revolution, when the National Convention on 27 October 1793 integrated the Principality of Monaco into the territory of France. This arrangement lasted until the enactment of the Treaty of Paris on 30 May 1814, which reestablished the principality under its previous borders and independent government by the Grimaldi family.
The second interruption occurred just shortly after as a result of the next Treaty of Paris on 20 November 1815, which made Monaco a protectorate of the Kingdom of Sardinia. This led to the Monegasque postal system becoming Sardinian from the signing of the Treaty of Stupinigi, which took effect on 1 January 1818, until the signing of the Treaty of Turin on 24 March 1860, again making Monaco a principality of France, and restoring the postal relationship.
Monaco created its first stamps in July 1885 under Charles III, although the country's postal service at the time was still technically French. The post office didn't became an independent subsidiary until 1937 under Prince Louis II. Monaco became a member of the Universal Postal Union in 1955, joined PostEurop in 1993 and became of founder member of the Small European Postal Administration Cooperation in 1999.
From Wikipedia’s article on La Poste Monaco, shared under CC BY-SA 4.0.4