RCW 46.87.040

Additional gross weight may be purchased to the limits authorized under chapter 46.44 RCW. Registration must be for the remainder of the registration year, including the full registration month in which the vehicle is initially registered at the higher gross weight. The apportionable fee initially paid to the state of Washington, reduced by the number of full registration months the license was in effect, must be deducted from the total fee due. A credit or refund may not be given for a reduction of gross weight.