N.H. Rev. Stat. Ann. § 358-D:3

A motor vehicle repair facility shall not perform any service or repair work set forth in the written estimate unless it receives the written permission of the customer to proceed; provided, however, that if it is impracticable to give the customer a written estimate or obtain his written permission to proceed, the motor vehicle repair facility may orally advise the customer of the items contained in the written estimate and obtain his oral permission to proceed.