Misled by BMW Montana

Good day

I am very disappointed in the service I got from BMW Montana Pretoria. I took my car for a service (320i 2010 model) and a quotation was sent to me after diagnosis. I gladly paid what was required of me and then they started with the service. Two days later I get an email from them saying that some parts also needs repairing and with the email, came an amount I was expected to pay. I then gave them a call and spoke to a lady named Fatima and wanted answered to why was this not detected during diagnoses that these parts needs repairing. She then transferred the call to her manager who goes by the name of buli or bullie ( didn’t get the name right) I asked him the same question and he tells me that it is usually the technician mistakes that they don’t pick up all that is to be fixed. I then asked him then how is that my problem and he was very rude to me and we went back and forth without any resolution. My question to BMW is, is this how you treat your customers and also if it’s a technician mistake, why is this affecting me cause I was given a quotation and I paid for what was required and then days later I am being asked for more funds so that my vehicle can fully function perfectly.