Disappointed and furious customer from Bedfordcentre Postnet

I am very disappointed at Postnet bedford centre . I went there to do outprints and elamination. I sent an attachment for printing out 12 items and the cashier printed 36. I am now forced to pay over 600. I went there to rectify this but she just told me that there's nothing they could do about this. I tried calling the head office but they not picking up .