Yes, POSTAGE is a Scrabble word!

Letter values in Scrabble US: P 3, O 1, S 1, T 1, A 1, G 2, E 1.

Is it a valid word?

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

the charge for mailing something
a small adhesive token stuck on a letter or package to indicate that that postal fees have been paid

Also: postage stamp.