Worst customer service ever!!

Worst customer service ever. I made a online payment that went through twice. Both showed pending. The one on the 13th went off and they shipped my order. On the 15th the second payment went off but didnt go through because there wasnt enough money in my cheque bank account. So the payment of the 15th shows pending and my bank account in arrears. I contacted fnb to ask them to cancel the second payment because its already paid and my package was shipped. Witch means Im paying double for the item I bought. Fnb refuse to cancel the pending faulty payment!!!!! So the minute that money is paid into my account the payment is going to go through. The worst part of all is that they keep sending me messages daily about my account being in excess but I did tell them whats going on and they just refuse to help. Keeps sending me dispute forms and says I must sort it out with the consumer but the consumer havent received the second payment yet. It should be cancelled at my bank before I put money in. Fnb customer service is poor and they let you phone and put you through from one person to the other for more then an hour!!! I am so disappointed in them I am considering changing from banks.