Follow the manufacturer’s instructions to assemble the opener and mount the rail to the header bracket above the door. Then set the opener on a ladder where you’re going to install it. The ladder (usually an 8-footer) holds the opener in position while you measure for your lengths of angle iron. If necessary, put boards under the opener to raise it.
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.