There is a HUGE part of the problem.
IF customers were informed of the potential for the need for additional work, it would have made a huge difference. Maybe there is NO mention of this in the parts description and NO mention of it at the time of sale.
Some MFers need to stick to making parts and let others handle the phone, you know....people with some decent social skills.
MOST of the time, a decent attitude can avoid problems and keep a customer. An arrogant asshole will piss off people and chase business away to his competitors.