Wholesalers and manufacturers, when selling to retailers or other persons, may charge a service charge of up to one and one-half percent per month on the remaining balance of all overdue accounts, provided the parties have entered into a written agreement prior to the transaction setting forth the amount of service charge, computed on the basis of simple interest per annum. The wholesaler or manufacturer shall inform the purchaser in writing at the time of the purchase of the service charge which will be charged if the account becomes overdue. The service charge allowed in this section is allowed on any such purchase on or after July 1, 1971.