Incompetency

I am a student who has a contract with virgin active .I was given in writing that my instalment would be R680. For the month of September I was debited the R680 and on the 3 October I was debited R1345 without being informed .After calling the customer service center they were very unhelpful and had no reason as to why I was debited R1345 and provided no solution to rectify the situation