I do automotive work for friends/family, and I only ever charge the part cost. Nobody ever came back and wanted their money back...

I once did a full engine replacement in a friend's car (just charged her parts) and it turns out I routed a coolant hose too close to the engine. It rubbed a hole about 6mo later and left her stranded. She had it towed to a dealer and they fixed it - she was out abotu $200 for the tow/work. When I offered to pay he half the cost of the tow/work she turned that down, reminding me how much money she saved on having me do the engine work (about $2k).