Washington Revised Code Title 19. Business Regulations--Miscellaneous § 19.52.060. Interest on charges in excess of published rates

Any corporation, partnership or individual who furnishes the public any goods, wares, merchandise, pledge, security, insurance or transportation of which the price, rate or tariff is by law required to be published, shall, when any price, rate or tariff is charged in excess of the existing and established price, rate or tariff, refund to the person, partnership or corporation so overcharge, or to the assignee of such claim, the amount of such overcharge, and on failure so to do, the claim for such overcharge shall bear interest at the rate of eight percent per annum until paid.