As you enter life you are going to find that car repairs can be an expensive proposition. That does not mean that you’re being ripped off.
How can they overcharge you? You were given an estimate up front. Don’t like the estimate then get another.
Failing to find one that you like then try to do the job yourself.
Many shops don’t like using customer provided parts for several reasons. One is that fitment can be a hit and miss thing with that part fitting correctly; or it may not fit at all without a wrestling match.
Two is that quality can also be hit and miss. The shop installs some drop shipper provided Chinese part from Wing Wang Motor Works and the shop will also be the one to catch the blame when that part fails 3 days after installation.