Amounts so assessed against any public service company not paid within thirty days after mailing of the bill therefor, shall draw interest at the rate of six percent per annum from the date of mailing of the bill. Upon failure of the public service company to pay the bill, the attorney general shall proceed in the name of the state by civil action in the superior court for Thurston county against such public service company to collect the amount due, together with interest and costs of suit.