The holder of a contract, upon request by the buyer, may extend the scheduled due date of all or any part of any installment or installments, or renew or restate the unpaid time balance of the contract, the amounts and due dates of the installments, and may collect for that extension, renewal or restatement a refinance charge computed as follows: If the unpaid time balance of the contract is extended, renewed or restated, the holder may compute the refinance charge on an amount determined by adding to the unpaid balance the cost to the holder for insurance and official fees incidental to the refinancing and accrued delinquency and collection charges and deducting any refund credit which may be due the buyer for prepayment under Section 7 at the rate of finance charge applicable to the net balance at the time of refinancing.