Over charged and very bad service

So I have been in contract Sim only deal with telkom for almost 3 years now, I have never skipped a payment not even once so it happend that on the 26th of May they tried to debit but there were no funds, okay I them made an Eft payment of R417 on the 29th of May and then on Saturday the 31st of May they debit R416 that I payed and then I said okay let me leave it so that it falls on the next months debit order.. So today I receive an invoice saying I am gonna pay R590 like how, and when I called I was answered by yoliswa from the contract enquiries department and when I ask her she says it's their system it is not their problem and I could have avoided this predicament if I payed on time, like how is this possible telkom and hire people whom can't even assist you and tell you this bull...I know you charge a ridiculous rejection fee by the way which doesn't make sense at all but in total I Paid R839 for one month and again you say I am gonna pay R590 how, so where did my other R416 go.