Tenn. Code Ann. § 47-25-1806

All parts used by the retailer in performing such warranty work shall be paid to the retailer in the amount equal to the retailer's net price for such parts, plus a minimum of fifteen percent (15%). This addition is to reimburse the retailer for reasonable costs of doing business in performing such warranty service on the supplier's behalf including, but not limited to, freight and handling costs incurred.