Over charged

I have a contract with telkom for phone and Sim card. When I open the account I was ask which day would I like them to deduct the money on my account which I said on the 25 of every month's. Last month it was my first payment and I didn't get money early that day so it bounce on the 26 I went to telkom and asked for the account on which I'll make the payment and I was assisted. I made my payment that very day on the 26. But this month I was told that I have to pay the contract with extra R300 for the penalty that last month they didn't get money on time. Guys I mean it was barely 24 hours now they are charging R300.