Wash. Rev. Code § 35.21.845
A motor carrier of freight for hire whose gross receipts are subject to multiple taxation by two or more municipalities in this state may request and thereupon shall be given a joint audit of the taxpayer's books and records by all of the taxing authorities seeking to tax all or part of such gross receipts. Such taxing authorities shall agree upon and establish a tax allocation formula which shall be binding upon the taxpayer and the taxing authorities participating in the audit or receiving a copy of such request from the taxpayer. Payment by the taxpayer of the taxes to each taxing authority in accordance with such tax allocation formula shall be a complete defense in any action by any taxing authority to recover additional taxes, interest, and/or penalties. A taxing municipality, whether or not a party to such joint audit, may seek a revision of the formula by giving written notice to each other taxing municipality concerned and the taxpayer. Any such revision as may be agreed upon by the taxing municipalities, or as may be decreed by a court of competent jurisdiction in an action initiated by one or more taxing authorities, shall apply only to gross receipts of the taxpayer received after the date of any such agreed revision or effective date of the judgment or order of any such court.
RCW 35.21.845
Applicability-1982 c 169: See note following RCW 35.21.840.