You must submit tax for merchandise you sold on credit (i.e. installment sales) in the reporting period in which you make the sale, even though you may not receive full payment until a later date. Therefore, you owe both retailing B&O tax and retail sales tax on the full selling price when you make the sale.
Example: You sell a $2,000 watch on credit in April and allow the customer to take possession of it immediately with a $500 partial payment. You would report the sale as taking place in April, even though you did not receive full payment in that month. You should report the entire $2,000 selling price in April.
You may exclude separate charges for insurance, interest and finance charges from the selling price when determining amounts subject to retailing B&O tax and retail sales tax. However, these charges are subject to service and other activities B&O tax (WAC 458-20-198).