Yes, PST is a Scrabble word!

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

Is it a valid word?

PST is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

standard time in the 8th time zone west of Greenwich, reckoned at the 120th meridian west; used in far western states of the United States

Also: pacific time, pacific standard time.