Yes, CASHIER is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, S 1, H 4, I 1, E 1, R 1.

Is it a valid word?

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

an employee of a bank who receives and pays out money
a person responsible for receiving payments for goods and services (as in a shop or restaurant)

Also: bank clerk.