Yes, ZIP is a Scrabble word!

Letter values in Scrabble US: Z 10, I 1, P 3.

Is it a valid word?

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

a quantity of no importance
a code of letters and digits added to a postal address to aid in the sorting of mail
forceful exertion
a fastener for locking together two toothed edges by means of a sliding tab

Also: goose egg, zip code, postal code, slide fastener, zip fastener, zip up, travel rapidly.

Not playable in Scrabble US: ZIPPO.