All fees and medical charges under this title shall conform to regulations promulgated, and the fee schedule established by the director and shall be paid within sixty days of receipt by the self-insured of a proper billing in the form prescribed by department rule or sixty days after the claim is allowed by final order or judgment, if an otherwise proper billing is received by the self-insured prior to final adjudication of claim allowance. The self-insured shall pay interest at the rate of one percent per month, but at least one dollar per month, whenever the payment period exceeds the applicable sixty-day period on all proper fees and medical charges.