Section 11. A. No fee, expense or other charge whatsoever shall be taken, received, reserved or contracted for, except as provided in this section and in sections sixteen to twenty-two, inclusive, and except for official fees, and for the items expressly provided for in the retail installment sale agreement as set forth in section nine.

B. An installment seller may, in a retail installment sales agreement, contract for, and if so contracted for the holder thereof may charge, receive, and collect a finance charge computed on the original amount financed of the contract or obligation not in excess of an annual percentage rate of twenty-one per cent.

C. On agreements payable in successive periodic installments substantially equal in amount, the maximum permissible finance charge shall be computed on the original amount financed. On agreements providing for installments extending for a period less than or greater than one year, the finance charge shall be computed proportionately.

The finance charge may be computed on the basis of a full month for any fractional month period in excess of fifteen days.

When a retail installment sale agreement provides for unequal or irregular installments, the finance charge shall be at the effective rate provided for in this section, having due regard for the schedule of installments.

The date on which the finance charge begins to accrue shall be deemed to be a different date from the date of the transaction if the goods or services purchased are not available for delivery on the date of the transaction and the date such finance charge shall begin to accrue shall be the date when such goods or services are available for delivery.

D. No installment seller shall permit any person, or any husband and wife jointly or severally, to be obligated, either directly or contingently to said seller, under more than one retail installment sales agreement executed at the same time.