account in arrears

I paid for March with no hassles. In April they tried debiting my account a day before my paydate and there was no money. I went back to the store and the details were amended and account was debited on the 30 April for R137.50. For May my account was not debited on my payday and I called the customer service and the lady told me that there was no attempt to debit my account at all. The case number was ***7 on the 28 May. Nothing happened since then and on the 11 June I get letter stating I am in arrears and I need to pay the amount of R137.50 which was already paid. I had to make another trip to the store and the manager could not explain how this happened. He called the call centre and they said that I must make manual payment.I am confused as the payment has already gone off . Further more I am listed in arrears for 30 days which will impact me negatively but I couldn't get an explanation as to what happened and all they requested is for payment to be made. I signed an agreement for R129 and was billed for R137.50 an additional cost added on that I didn't agree to. This has become so frustrating as I seem to be wasting my time and effort on problem created by Telkomobi.