Pursuant to authority granted by RCW 15.66.170 and by WAC 16-516-020(8), in the event that any assessment is not paid within 90 days after the date of the billing therefore by the commission, or within 90 days after the due date of the report required by WAC 16-516-110 (b)(iii) and (iv) a sum equal to 10% of such unpaid assessment of unpaid portion thereof shall be added thereto and be due and owing to the commission.