Prior to the effective date of any contract or arrangement described in RCW 80.16.020, each gas utility must file a verified copy or a verified summary, if unwritten, of contracts or arrangements, except for transactions provided at tariff rates, with any affiliated interest. Prior to the effective date of any modification or amendment, the utility must file verified copies of the modifications or amendments to the contracts or arrangements. If the contract or arrangement is unwritten, the utility must file a verified summary of any modification or amendment. The commission may institute an investigation and disapprove the contract or arrangement if the commission finds the utility has failed to prove that it is reasonable and consistent with the public interest.