When I work on something for a customer, I always offer them the old parts back, minus fluids. But then i pull the dipstick if so equiped, or visually make sure that they see fluids are new, clean and full. Be it i show them the oil sight glass, removing the rad cap, etc. Some customers also sit and wait for me to do the repairs if just a quick job.
My uncle took in his pickup for a bunch of work last year. They charged him $1,807 and did nothing to his pickup except for changing the engine oil filter, and engine oil. He sued them in court, won $15,000 and the best thing was the dealership owner and 2 employees in court, answering to the judge for their fraudulent actions.