(1) Customer bills must:

(a) Be issued at intervals not to exceed three months and identify if the water company is billing in arrears or advance;

(b) Show a reference to the applicable rate schedule;

(c) Identify and show each separate charge as a line item;

(d) Show the total amount of the bill;

(e) Include enough information that, together with tariff rates, the customer can calculate his or her bill (a copy of the tariff is available for review at company or from the commission upon request);

(f) Show the date the bill becomes delinquent if not paid. The minimum specified time must be fifteen days after the bill's mailing date, if mailed from within the state of Washington, or eighteen days if mailed from outside the state of Washington, after the bill's mailing date.

A customer may request to pay by a certain date that is not the normally designated payment date when showing good cause. Good cause may include, but is not limited to, adjustment of a billing cycle to parallel receipt of income. The preferred payment date must be prior to the next invoice date.

(g) Include the water company's name, business address, and telephone number and/or emergency telephone number by which a customer may contact the company;

(h) If the customer is metered, include the current and previous meter readings, the current read date, and the number and kind of units consumed;

(i) Show taxes and any tax percentage rate that the taxes are computed from. Taxes must be totaled to show a total taxed amount. Upon request, the company must provide a detail of the computation of the tax amount. Taxes, as used here, represent municipal occupation, business and excise taxes that have been levied by a municipality against the company, and are being passed on to the customer as a part of the charge for water service; and

(j) Clearly identify when a bill has been estimated.

(2) Each water company may prorate bills for customers who have taken service for a fraction of the billing period. If the company does not have its method of prorating bills in its tariff, the company must prorate bills in the following manner:

(a) For flat rate service, the charge must be prorated on the basis of the proportionate part of the period during which service was rendered.

(b) For metered service the charge will be equal to:

(i) The applicable minimum charge as shown in the company's tariff must be prorated on the basis of the proportionate part of the period during which service was rendered; plus

(ii) Any water usage charge computed using rates and allowances shown in the company's tariff.

(3) The water company must include its method for estimating bills in its tariff. Estimating of bills is allowed for no more than two consecutive billing cycles.

(4) When a company has cause to back-bill a customer, the company must allow the customer payment arrangements, if requested, for the same number of months to pay equal to the cumulative total of months being back-billed. (Example: If the company is back-billing for a one-year period, the company must allow the customer twelve months of equal payments to pay the total amount of the back billing.) These payments will be in addition to current billings.