Back in the day, mechanics would "pad" the bill with added charges for services that the customer did not need or ask for. This led to the 10% allowable overage that became a standard for the repair business.

I believe that most shops are honest and want to do right by their customer. There are a few shops that I know of personally that partake in the practice of overcharging the customer. It just becomes common operating procedure for them and they continue to "steel' from their customers.

I have been in business for a long time and I strive to always give true and accurate estimates that are within pennies of the final bill. If an unforeseen expense comes up, as they occasionally do, I will call the customer and explain to them what I found and exactly what the added cost will be before I proceed with their project. This will eliminate any dispute with the customer when they come to settle the bill.
After all, honesty IS the best policy.

In one of the very first shops I worked in doing upholstery, the owner would always add at the bottom of the estimate...'Shop supplies...$xx.xx.'
It always seemed worked out to be just around that 10% margin. He explained that that was to cover things like hog-rings, glue, staples, etc, etc.
Never seemed right to me. A bit of a money grab. Padding the bill. Whatever.
I don't believe in that.
My view is that those 'supplies' are part & parcel of doing the job. If the estimate was done correctly, then there wouldn't have to be that caveat added on the end of the bill.

I have been in business for a long time and I strive to always give true and accurate estimates that are within pennies of the final bill. If an unforeseen expense comes up, as they occasionally do, I will call the customer and explain to them what I found and exactly what the added cost will be before I proceed with their project. This will eliminate any dispute with the customer when they come to settle the bill. After all, honesty IS the best policy. — Fred Mattson

Amen to that!