A special rebate of precomputed charges shall be made if three or more, but not all, installments are prepaid in full at any one time either in one transaction or over a period of time on a loan made under Section 18636. The special rebate shall be equal to the portion of precomputed charge applicable to the last installment period multiplied by the total number of full installment periods such installments and any subsequent installments are prepaid. Such special rebate shall be computed and made at the termination of the loan contract and shall be an addition to any required rebate for prepayment in full.