(a) Payments of first party benefits and additional first party benefits shall be made as the loss is incurred. Such benefits are overdue if not paid within thirty days after the claimant supplies proof of the fact and amount of loss sustained. If proof is not supplied as to the entire claim, the amount which is supported by proof is overdue if not paid within thirty days after such proof is supplied. All overdue payments shall bear interest at the rate of two percent per month. If a valid claim