(b) All work done by a farm machinery repair shop, including all warranty work, shall be recorded on an invoice, which shall describe all service work done and parts supplied. If more than one act of maintenance or repair is performed by a farm machinery repair shop, the invoice shall be written in such a way that the labor cost per hour and total labor cost, as well as the specific parts used and their cost, shall be recorded on a per-job basis. However, where work is done on an agreed total-cost-per-job basis, or the work includes an agreed total cost for component unit replacement, the invoice shall describe the work done on such basis and the total cost for such work.