Wash. Rev. Code § 49.52.060
The provisions of RCW 49.52.050 shall not make it unlawful for an employer to withhold or divert any portion of an employee's wages when required or empowered so to do by state or federal law or when a deduction has been expressly authorized in writing in advance by the employee for a lawful purpose accruing to the benefit of such employee nor shall the provisions of RCW 49.52.050 make it unlawful for an employer to withhold deductions for medical, surgical, or hospital care or service, pursuant to any rule or regulation: PROVIDED, That the employer derives no financial benefit from such deduction and the same is openly, clearly and in due course recorded in the employer's books.
RCW 49.52.060
Penalty for coercion as to purchase of goods, meals, etc.: RCW 49.48.020.
Public employment, payroll deductions: RCW 41.04.020, 41.04.030, 41.04.035, and 41.04.036.
Wages to be paid in lawful money or negotiable order, penalty: RCW 49.48.010.