Remind them that they authorised the repair and it's not subject to negotiation after the work is completed. Without payment i will see you in Small Claims Court. I personally won't touch but only what i am called out to repair because of these situations that can arise from them. Word of advice to all of you. Signed Contract, Make the Agree'ed Upon Repair, Notate other needed repairs on the bill, Collect and LEAVE!!! No favors or good sameritan stuff...Simply do as the customer asked (right or wrong) and Leave...Sad but True...Chalk it up as a learning experience.