Underquoted for cash, now the nurse has to pay the difference

I was quoted for a set of blood tests as I am a cash customer. I paid the amount quoted to then receive an invoice telling me I hadn't paid the full amount. Long story short, the nurse didn't use the correct blood test code and I am now in for 1,200 extra which has been revised to R860. I paid for a service and I expect that what I paid for is undertaken. Not for a different test to be done with the automatic assumption that I will just pay up. On discovering that there was a price difference my expectation is that I WILL BE PHONED and advised so I can agree to the new amount. This did not happen. I am unable to get this resolved and the nurse has been handed the burden and is under the impression that as she loaded the incorrect item she will have to pay if I do not! WHICH IS TOTALLY UNACCEPTABLE. The processes in place are obviously deficient and now I am suddenly REQUIRED to pay for something I DID NOT BUY and if I do not, the nurse has to pay ?????? !!!!!!