One of these "sproing" events at our house finally motivated me to research how these repairs are done. This happened in 2002, when my wife parked the chariot and shut the door. After the door closed, there was a horrific noise that she could only astutely describe as, "a big spring snapping and vibrating". Although I have hired professionals several times in the past to install or repair garage doors, the difference this time was the innovation of Google and newsgroups like alt.home.repair. I was determined to learn the process and to search for online parts vendors.
You might genuinely need some extra parts when you thought you simply needed a broken spring replaced, and a good serviceman will perform a simple inspection to identify such parts. Nor is it unreasonable for a business to charge separately for a service call versus repair work actually performed. But the best protection for you as a buyer, being somewhat at the mercy of whomever you decide to bring on site, is to understand what is being done, and ask intelligently for a clear explanation or demonstration of why extra parts are required.