Postnet make a mistake a charge the customer for it

I sent an email to Postnet Lyttelton asking them to make two A3 copies of a building plan. When I arrive I happen to take a phone call. during this phone call the guy behind the counter apparently asked me if I would like the plans printed on A2. I did not request A2 nor did I respond to his question. He then printed the pages on A2 and charged me R220 instead of R46 that it would have been had he printed on A3. they made a mistake, admitted to it and still refused to give me a refund. I spoke to the manager to had a terrible attitude and shouted at me saying it was my fault before returning to her office.