A custom upholsterer shall give to his or her customer a written estimate of the price of the labor and materials necessary for a specific job. No work shall be performed and no charges shall accrue before authorization to proceed is obtained from the customer, and no charge shall be made for work performed or materials supplied in excess of the estimated price without the oral or written consent of the customer obtained after it is determined that the estimated price is insufficient and before the work not estimated is performed or the materials not estimated are supplied. Nothing in this section shall be construed as requiring a custom upholsterer to give an estimate if he or she does not agree to perform the requested work. As used in this section, “materials” includes structural units, filling materials, containers, and coverings.