Surely this is about the breakdown of a business relationship.
I took my car to the same garage for years, then began to believe that they were overcharging so I moved garage.
This situation is not unusual. If they are overcharging so badly the word will get round and they will have no customers.
It is a commercial situation, they can charge what they want.
As long as they do not artificially inflate hours on a job there is no problem.
As they are quoting up front this is not the case.
No one is being forced to use this maintenance organisation.
Anybody who has tried to book a holiday when the kids are off school has similar issues, the price suddenly jumps. Market forces are at work.
To be honest I have little sympathy. I have worked in G.A for over 30 years often earning a pittance and watching maintenance companies charging less than the local garage. I remember one helicopter owner who's car tyres each cost more than I was paid a week.
I've watched maintenance outfits reduce legitimate bills after the owner has come in whining about the cost, because they didn't want to loose the business. Done that myself in fact. For the first time G.A maintenance is in the position to make a profit and pay their staff a decent wage.
If you are that unhappy move on, there is nothing worse than believing you are being ripped off.