For the purpose of this section, "periodic time balance" means the unpaid portion of the time balance as of the last day of each month, or other uniform time interval established by the regular consecutive payment period scheduled in a retail installment contract.

Notwithstanding the provisions of any retail installment contract to the contrary, and if the rights of the purchaser have not been terminated or forfeited under the terms of the contract, any buyer may prepay in full the unpaid portion of the time balance thereof at any time before its final due date and, if he or she does so, he or she shall receive a refund credit of the unearned portion of the service charge for such prepayment. The amount of such refund credit shall be computed according to the "rule of seventy-eighths," that is it shall represent at least as great a portion of the original service charge, as the sum of the periodic time balances not yet due bears to the sum of all the periodic time balances under the schedule of payments in the contract: PROVIDED, That where the earned service charge (total service charge minus refund credit) thus computed is less than the following minimum service charge: Fifteen dollars where the principal balance is not in excess of two hundred and fifty dollars, twenty-five dollars where the principal balance exceeds two hundred and fifty dollars but is not in excess of five hundred dollars, thirty-seven dollars and fifty cents where the principal balance exceeds five hundred dollars but is not in excess of one thousand dollars, and fifty dollars where the principal balance exceeds one thousand dollars; then such minimum service charge shall be deemed to be the earned service charge: AND PROVIDED FURTHER, That where the amount of such refund credit is less than one dollar, no refund credit need be made.