Charge more than I was told to pay

Mid June I WhatsApp. in connection with an upgrade and the person told me about uncapped that cost R399. I was asked when must it be started and I sail immediately and was told that, that cannot be done it will start on 1/07 of which I agreed. On 01/07 an amount of R491 was debited and when I enquire about it I was told that it was a special that ended end of June and it lasted for 3months which was May and June and when I upgraded I WAS NOT INFORMED