Incorrect billing and overcharge

I took a contact on special which was R399 for six months and R609 thereafter. I saw a the amount reflecting on the email.I called and confirmed before I activated the connection at home and lady I spoke to said I should ignore that email.They will debit R399 for six months.Today 31 they tried to debit R851..How ?I completed the order online and paid the R250 installation ,so what is the R851.The internet was used from the 1st of Dec