(c) Each payment received after the subsequent purchase shall be deemed to be allocated to all of the various time balances in the same proportion or ratio as the original cash sale prices of the various retail installment transactions bear to one another: PROVIDED, That the seller may elect, where the amount of each installment payment is increased in connection with the subsequent purchase, to allocate only the increased amount to the time balance of the subsequent retail installment transaction, and to allocate the amount of each installment payment prior to the increase to the time balance(s) existing at the time of the subsequent purchase. The provisions of this subsection shall not apply to cases where such previous and subsequent purchases involve equipment, parts, or other goods attached or affixed to goods previously purchased and not fully paid, or to services in connection therewith rendered by the seller at the buyer's request.