The customer could have walked at any time.
There may not be another part of the store that is accessible to customers. Often security concerns and insurance restrictions dictate "customer areas".
Brownells polls gun shops for their typical shop charges every so often:
https://www.brownells.com/userdocs/miscellaneous/shoppricesurvey.pdf
$85 does not seem excessive. If it does, then the young man better learn how to install an AR grip himself.
Not to mention repairs performed "while you wait" are almost always priced at a premium.
Then he should have asked. Often its not possible to get a "cost of service" for repair work until the repair is complete. My plumber doesn't do it, my A/C guy doesn't, my tech at the car dealership doesn't. They may give a range, but rarely an exact price.
How do you figure he was overcharged?
Often the cost charged isn't for the small, easy repair.....but because the gun shop has to keep a gunsmith employed. Having someone available costs $$$ whether there are customers or not.
When I do transfers, my transfer fee isn't calculated by how much time I spend with the customer when he comes and picks up. He may be here five minutes tops. My transfer fee is charged
because I hold an FFL. Unpacking the firearm, recording the acquisition on my bound book and preparing the 4473 before the customer arrives is only part of what I calculate in setting my fees. Yet I know every day people leave here thinking Tom just made $20 for five minutes work.
They don't see my alarm bill, my dealers inventory insurance, the time spent maintaining my records or anything else.
Not only was the charge not excessive, you have no idea if the store made a profit. You saw one transaction. You have no idea if the store sees enough repairs to cover the expenses of employing a gunsmith or clerk to perform repairs.
Which is irrelevant.
1. Customer came to them, for a service they charge for.
2. The cost of the repair was normal.
3. How long it took....has no bearing.
4. Being ignorant may be expensive. Learn how to do it yourself. No one forced the kid to bring the gun in. He easily could have asked a friend, watched one of a thousand YouTube tutorials or read the freaking instructions on the box.
You should run them out of business doing free gun repairs.
Seriously, if the kid bought the grip there, they would likely have done the install free. Happens all the time with night sights. But good grief, the kid likely bought his grip and reflex sight from Amazon and you expect the shop to provide free labor? Wow.
I think your conclusions about the shop, their fees and the entire situation is out of line.
Be treated politely, attended to promptly and being thanked for my business. Rude clerks means you dont get my business.