Bad service

Last month I called Mtn to change my debit order date from the 25th to the last day of the month however they debited twice (25th and 31st) I called in informing them about the double debit and everything was sorted thereafter I reversed one debit Yesterday I received an email saying that I will be debited R518 on the 31st instead of R259 and I was told I owe the month of the March and they can't see the debit for the last day of the month they requested I send them a statement to prove I paid