RCW 50A.45.015

At any time after the commissioner shall find that any premiums, interest, or penalties have become delinquent, the commissioner may issue an order and notice of assessment specifying the amount due, which order and notice of assessment shall be served upon the delinquent employer in the manner prescribed for the service of a summons in a civil action, or using a method by which the mailing can be tracked or the delivery can be confirmed. Failure of the employer to receive such notice or order whether served or mailed shall not release the employer from any tax, or any interest or penalties thereon.