In the case of where you have bought the parts, and found the job beyond your scope, then here is where having a decent relationship with your mechanic comes in.
Explain the problem, and work something out where you can use the parts and he/she profits on the job.
In the electronics biz, we had a higher labour rate for installing customer supplied bits. We found that most (99%) people bringing their own parts had tried to install it themselves, and botched the job. Then, we had to sort out the goof and redo everything. What a hassle!
I had one kid bring in his car for an amp install. He had mounted the thing, run the cable, and hooked up the speakers. He simply wanted us to install an inline fuse and hook it up to the battery. Why? This made no sense to me. Well, he figured it was a five minute job, and should pay 5/60 of an hour's labour.
I explained that before we would do such a task, we would have to verify all the other connections, test the amp and the completed set-up. We had a .5 hour minimum charge, and I thought it would run about 1 hour's labour in total. He FREAKED! He began screaming at me that I was a rip-off artist and should be charging him $3.75 for the job. Five minutes at $45/hour!
Then, he wanted me to PAY HIM for the gas money he wasted driving to my shop. He began to look like he was feeling kinda froggy, so I asked him if he would mind leaving the showroom before we had to eject him.
He seemed to get the idea that I was about to squeeze all his zits at once, and left.
We got at least one a week of morons like that.
OTOH, I had fella that liked to buy his gear from the Bargain Finder, and have us install it. However, he bought all his wiring, kits, and fabrication work from me. He paid the true labour cost, and we did excellent work for him. In the end he did save quite a bit of money, and rewarded us with lots of referrals from his friends, most buying their gear from us. It was a good relationship that worked for both sides.
We were a good shop that gave good customers excellent service. Come through the door expecting me to pay a tech good money while you throw $3.75 on the counter, and I would skid you out the door without a thought. Good riddance.
1998 C230 "Black Betty"