Debt collection frustration

I had a debit order of R470 that bounced at the beginning of the month. A few days later I paid the amount into their account. Then I received a collection letter stating that I owe R980. Where did the extra R510 come from?? Now I am being hounded for this money which I refuse to pay, as I have paid the initial amount that bounced. I am incredibly frustrated with this as in the 13 years that I have had this account this is the first time this has happened and I'm being treated as if I owe thousands of Rands.