What if, at an inspection, a mechanic happens to identify that additional parts are needed to fix the car?

In this case the mechanic will give you a quote for the labour and parts that are needed to address the problem. If you agree, the mechanic will endeavour to get the parts and return at another time you agree to. In some cases, due to non-availability of parts in your area, this might mean that the mechanic will not be able to return until another day.