LBS ordering their parts from the wholesalers also get free shipping when the order exceeds X $ so they can buy quite a few parts at no shipping whereas if you or they have to buy it as a 'one off' item they have to pay shipping and the customer should bear that cost burden.
That is how we do it at the shop I work. Most are OK with that if the alternative is having to wait several weeks or months to get the part from our usual sources.
We also install parts brought in from our customers. Sometimes that particular part is hard to get and requires a lot of internet searching which means a shop may be paying someone who spends an hour or two trying to find a part. We run into that a lot on older mech hangers and especially some ebikes which are internet only buys.
I recently did a bike rebuild converting a mech bike to a Di2 11 speed. The customer bought all the parts online and brought them to us. We had to strip the bike down to the frame and rebuild it with his parts. He made a few mistakes that cost him both time and money. He bought mech brake shifters instead of hydro so which he decided to keep so he had to buy mech calipers. He bought the wrong battery and couldn't find the correct one online but lucky for him we had one in stock. He also bought the wrong length wires, several of them were too short. Again lucky for him we had some in stock and also a connector.
He wound up spending more than he budgeted for because of his mistakes, which we would have avoided for him if he went through us from the beginning even if we had to buy the parts singly and pay shipping. Some of his parts were purchased from England and the shipping to send them back isn't worth it so he is going to sell them on ebay...losing money. Add to that the several phone calls from us telling him another part wasn't correct was disheartening for the customer.
In the end the build was completed successfully and works like a dream and the customer is super satisfied but it was not easy for him or us.
It also took me more time than it should have because I had to stop to figure out what was wrong, search to see if we had the part, call the customer to speak about the problem, look for the part online unless the customer did it, wait for the part, etc.
Builds like this are now going to cost more because the shop can't afford to pay me an extra hour or two overall when the margins are small to begin with. This shop lost money on this build because we stuck to our standard rates but we won't do that again.
This happens a lot in local shops because most customers, even knowledgeable ones often order the wrong part and we get stuck with the lost time having to sort it out and make it right. Our rates are now going up to reflect this.
You certainly don't get away with it when bringing your car in for work especially upgrades, you pay by the hour and it is not cheap.