Poor customer experiance (Overcharged)

I have be a loyal customer who worked with previous manager very well when they were fixing my cars and they gave me a good price at car service at westgate branch . Now here come this new manager by the name of Andrea who called me to come service my car, so I went there to give them business, so he gave me a quote that he will do a major service on my car for R1637. and on my invoice he over charged me with an amount of R681. So when I went there to enquire about it he gave me an attitude that he cant help me and the invoice is done already it was their admin lady's mistake.. so I felt like am nobody after spending 3years with them servicing my car with Car service there at westgate. I just feel like this Andrea guy if he keep on doing like this overcharging customer, he will drive the business down the drain. And am still going to take this issue further more.