Yes, POST is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, S 1, T 1.

Is it a valid word?

POST is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the position where someone (as a guard or sentry) stands or is assigned to stand
military installation at which a body of troops is stationed
a job in an organization
an upright consisting of a piece of timber or metal fixed firmly in an upright position
United States aviator who in 1933 made the first solo flight around the world (1899-1935)
United States female author who wrote a book and a syndicated newspaper column on etiquette (1872-1960)
United States manufacturer of breakfast cereals and Postum (1854-1914)
any particular collection of letters or packages that is delivered
a pole or stake set up to mark something (as the start or end of a race track)
the system whereby messages are transmitted via the post office
the delivery and collection of letters and packages

affix in a public place or for public notice
publicize with, or as if with, a poster
assign to a post; put into a post
assign to a station
display, as of records in sports games
enter on a public list
transfer (entries) from one account book to another
ride Western style and bob up and down in the saddle in rhythm with a horse's trotting gait
mark with a stake
place so as to be noticed
cause to be directed or transmitted to another place
mark or expose as infamous

Also: military post, wiley post, emily post, emily price post, c. w. post, charles william post, mail service, postal service, put up.