Yes, WINDFALL is a Scrabble word!

Letter values in Scrabble US: W 4, I 1, N 1, D 2, F 4, A 1, L 1, L 1.

Is it a valid word?

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

fruit that has fallen from the tree
a sudden happening that brings good fortune (as a sudden opportunity to make money)

Also: gold rush, manna from heaven.

Not playable in Scrabble US: BUNCE.