All repairs come with a 33-day warranty.

You finish a repair and the customer finds a problem three weeks later. Is it your fault or did something else break? The warranty says if it fails within 33 days and it's on you, you go back and fix it. You get paid for that rework, just like it's a new job.

This protects you because the terms are written down. You know what you're responsible for and what you're not. If a part breaks because it was defective when you installed it, that's covered. If the customer cracked it by dropping something, that's not.

Parts that break during rework also count. You're not eating the cost of a part that was already faulty. The repair shop budgets for this, and it's tracked so nobody's guessing.