(1) Premiums must be paid quarterly. Each payment must include the premiums owed on all wages subject to premiums during that calendar quarter. Payments are due to the department by the last day of the month following the end of the calendar quarter for which premiums are being paid.

(2) Payments made by mail are considered paid on the postmarked date. If the last day of the month falls on a Saturday, Sunday, or a legal holiday, the premium payment must be postmarked by the next business day.

(3) Premium payments are due within ten calendar days when a business is dissolved or the account is closed by the department. Premiums not paid timely are delinquent and subject to interest under RCW 50A.45.025.