For jobs over $1,000 we add a 10% contingency on estimate. We explain to the customer what it’s for (additional labor for broken bolts, old brittle clips, a seal that was not put on estimate, etc) and that if it’s not needed, it comes off the bill.
We use some of the contingency 20% of the time. If we sell it up front, the customer is usually resigned to the fact that the repair will cost the higher amount and when the job goes smoothly and we don’t need it, we tell them it’s less, and they are ticked pink.
i am also wary of putting up warning signs and the vibe they give off.
just my 2 cents