1. the US average is not meaningful in Texas
2. $75 for an oil change including filter and synthetic oil is not an overcharge
3. I am more than able to change the oil on my vehicle myself, just as I'm able to adjust the derailleur cables on my bicycle and mow and edge my own lawn. Ask yourself whether you are overcharging yourself to do these things
I can put all of my bicycles on the stand and replace frayed cables and crimp aglets onto the ends, or I can put every single bicycle I own into the back of the truck and have the bike shop do it for me. It's not a question of whether I've improved myself, it's not a Don Sharp "Under the Hood" question whether I respect the work involved. It's a time and annoyance issue best left to people who earn less than I do.
If I'm dissatisfied with the pull on badly-run brake and shifting cables, or badly installed cable housings, I can complain and have them replaced. If the mistake is my own, I am left with a dilemma: How important is it to me, in terms of blood and treasure, to fix the problem myself? This creates a schism in my thinking about priorities and commitments.
Bike shops do not have this problem. They source parts in large quantities, they perform these tasks with regularity, and the customer is always right when he says "I want the featherweight Shimano DA cable, just get it from QBP or whomever, but do it."