It's easy to glance at the total on a shop estimate, decide if it feels okay, and sign.
That number is a starting point, not a fixed price. Ask for a written estimate that requires your OK before the shop goes past it. A few lines on it are worth a second look before you do.
Labor
This is usually the biggest line, and the one people misread most. Many shops don't bill for the actual clock time a tech spends on your car. They bill a flat-rate number from an industry time guide (Mitchell, ALLDATA), multiplied by the shop's hourly rate. A tech who finishes in ninety minutes still bills the guide's number. One who hits a stuck bolt and burns three hours bills the same number. That's not padding by itself. It's how the system works.
What to ask: "What's your hourly rate, and how many hours are you billing for this job?"
Parts
Shops mark up parts. That's normal. They're sourcing the right part, stocking it, and standing behind it if it fails.
What varies is which kind of part you're getting. OEM parts (the same ones your car came with) are made by or for the manufacturer. Aftermarket parts are made by a different company to fit the same spot, and quality swings more by brand. For most routine work, brake pads, alternators, water pumps, a recognized aftermarket brand (Bosch, Denso, ACDelco, and similar) is usually fine. For parts that need a precise factory fit, sensors, seals, steering and suspension components, OEM tends to be worth the extra cost.
What to ask: "Are these OEM or aftermarket, and what brand?"
Shop supplies
Almost every estimate carries a line like "shop supplies" or "environmental fee." It covers rags, fluid disposal, and cleanup. Small and standard on most jobs.
It's also one of the easier lines to question, especially on a job that doesn't touch chemicals or fluids.
What to ask: "What's included in the shop supplies charge?" Some shops explain it, some just adjust the number. Either answer is fine, as long as you got one.
The line that shouldn't always be there
A diagnostic fee makes sense when the shop has to figure out what's wrong: an intermittent noise, an electrical gremlin. It's a different thing when you already know the problem and the shop still has to inspect it to scope the repair. Brakes are the clearest example. Grinding brakes still need someone to pull the wheels and measure the pads and rotors before they can tell you what parts you actually need. That's real labor, not a diagnostic re-charge.
What's worth questioning is a diagnostic fee stacked on top of full labor for work that overlaps it, or a charge for "diagnosing" something you already described in detail with nothing to show for it. Ask directly: what did the inspection find, and does the fee apply toward the repair if you approve it?
(If the brakes are actually grinding, stop driving on it and get it looked at right away, regardless of what the estimate says. That one's not optional.)
You don't need to question every line. You need to understand what you're signing before you sign it.
One question tells you a lot about a shop: "Can you walk me through this line by line?" A shop that does it without hesitating is a shop worth using. One that gets vague or defensive is telling you something too.
If the total still feels off after that, a second estimate from another shop is a fair next step. Just compare the same things, parts type, labor hours, warranty, not only the number at the bottom.