Airtime ****

I have recently bought airtime via their app. I wanted to buy R50 airtime but to my suprise 300 was deducted to my account. Then when I enquire about it they say there's nothing they can do about it, I try to explain that money is for transport as my balance was R450 its a mistake. I only needed R50 airtime now I'm left without money for transport and R300 airtime in phone that I don't want to use. How can they say they can't reverse the airtime its a day light *******. I'm passed off