Taking money that has already been paid!

We cancelled our line at the end of March. April was supposed to be the last invoice. Then we were billed in May for calls made in April. I reversed the debit order and asked for clarification as to why more money was deducted. Today I received an email explaining it was backdated, and that the R108 was the last amount. I paid and sent through conformation today. Now I received another invoice for R220 that will be deducted via debit order. After I paid via EFT. And why is it more now? Not very happy!