In Illinois if there is no agreement between parties, the plaintiff is only responsible for the first $100.00 of an upcharge from the initial agreement for a repair. This is the basis of the requirement of a signed work order in various businesses I've been involved with. However, if you as the repairing entity call and gain approval over the phone for the additional work or expense, this is allowed. Judges can be quite fair in automotive type repairs to both parties I've found. Just don't assume it will be alright and go without approval is the best way to do things.
I'm only citing automotive repairs in the above example but that law extends to most any service agency. I seen it applied just after the first of this new year but wasn't involved with it. Kind of got a chuckle from it too as one of those "I told you so" type affairs.
I did a job for the grandson of a very famous man in US history. It was a two car garage. I worked for the builder, but billed the owner. The project grew to a three car garage with a boiler room, gymnasium, bathroom on one level. Might as well have a second floor! A mortice & Tennon timber frame formed the large second floor. It became a guest apartment & music room. When it came time to settle up, he did not respond for 30 days when contractor & subs started getting nervous. Plumber showed up at his door to collect with his three sons & other members of the crew. He got paid in full. I filed a lien with the town. Town Clerk sat on my paperwork until he had time to transfer ownership of the property to his brother. My lawyer was able to prove my lien was delivered prior to his transfer of ownership. The lien had to be amended to include the brother. Cost me $800. in legal fees, but I was eventually paid in full. At one point he made the statement: "I own properties in 5 states, I pay my lawyer a handsome amount to assure me I don't need to pay a cent I don't want to.
Did a project for a new owner of a large house my father & I wired in 1974. It began as a kitchen renovation. It grew from that step by step over 2 years to a total renovation of house, pool house, and guest house/4 car garage. The whole renovation totalled $300,000. I billed monthly, & was paid promptly. In the chaos of the last summer the bill got up to $14000 he owed me. Last bill he ignored. Attempts to collect failed. I filed a lien, this time using a different lawyer. Lawyer advised I'd need to file a lawsuit within 6 months or the lien was void. He was confident the two judges who might be selected both took a dim view of deadbeat millionaires who don't pay. No counter claims were made, he didn't complain about workmanship, only refused to pay.
Turned out his lawyer had more pull than expected. She claimed me & my lawyer were well known in my county, the judge wouldn't be fair. Got the case moved to a different county where the judge was sympathetic to her. My lawyer was elderly & he got scared. He told me: "We could lose this case entirely. His lawyer offered a settlement of just over half what he owes. I believe you should take it."