A credit union may lend to its members at reasonable rates of interest, which shall not exceed one and three-fourths percent (1-¾%) per month, computed on unpaid balances, or invest the funds accumulated as herein provided. Fines and penalties shall not be considered as interest. A charge of Ten Dollars ($10.00) in lieu of interest may be made on any loan payable in a single payment, and a charge of Fifteen Dollars ($15.00) in lieu of interest may be made on any loan payable in installments.