Unfait penalty fee charge / *********** service

I went to the store to make a cash payments for my monthly account that is usually debited from my account 6 days before debit order date when i got to the store the consultant was helpful took my money and gave me my receipt on the devit order date a debit order went through and it was dishonored. I called vodacom help center to ask why a debut order went through on my account as i already paid and she informed me that she can see the payment on her side and they would probably not charge me a bouncing fee as they already had the payment before the debit order. 15 days later i got a billing call telling me i need to pay my monthly amount including. Penalty fee i contested the penalty fee due to the fact that when i called vodacom they based everything on my contract that i breached my contract by making a cash payment when the agreement stated debit order and as a person with business law experience i told them that vodacy assisted me in breaking that contract as they did not tell me this information when i was paying they gladly accepted my money with a smile then 6 days after suddenly im at fault and i need to pay them for paying my account early. I logged a query contesting the penalty fee they rejected the query saying I’m liable to pay the extra amount ... stating that their not at fault it is all my fault for paying cash. My confusion is i paid to an actual human being that signed the vodacom code of conduct with sales experience why not tell me that there will be consequences for paying my account in cash thus, she cannot accept my amount or i will have to inform the billing department if i do. They cannot answer that. But still give me an *********** and an ******* reason saying i breached a contract wjereas vodacom was the exact outlet that accepted my payment in the first place. At this point i more than dislike vodacom i would rate it a negative if possible If you are looking for any type of contract avoid vodacom i learnt the hard way.