(1) Personal protection insurance benefits are payable as loss accrues.

  (2) Subject to subsection (3), personal protection insurance benefits are overdue if not paid within 30 days after an insurer receives reasonable proof of the fact and of the amount of loss sustained. Subject to subsection (3), if reasonable proof is not supplied as to the entire claim, the amount supported by reasonable proof is overdue if not paid within 30 days after the proof is received by the insurer. Subject to subsection (3), any part of the remainder of the claim that is later supported by reasonable proof is overdue if not paid within 30 days after the proof is received by the insurer. For the purpose of calculating the extent to which benefits are overdue, payment must be treated as made on the date a draft or other valid instrument was placed in the United States mail in a properly addressed, postpaid envelope, or, if not so posted, on the date of delivery.

  (3) For personal protection insurance benefits under section 3107(1)(a), if a bill for the product, service, accommodations, or training is not provided to the insurer within 90 days after the product, service, accommodations, or training is provided, the insurer has 60 days in addition to 30 days provided under subsection (2) to pay before the benefits are overdue.

  (4) An overdue payment bears simple interest at the rate of 12% per annum.