On Line Bill Pay

I had set up an Online payment on Dec. 30th, 2010 to come out of my checking account on Jan. 4th, 2011. Some unexpected expenses caused me to xchange the pay date to Jan. 10th. The change was made on the 2nd. The money came out anyway on the 4th. I called the bank and they said they had no record of a payment for the 10th. My account was overdrawn, but they did reverse the overdraft payments. On the 10th the payment was taken out again. When I called the bank I was told that the problem was all my fault did not reverse the overdraft charges. I thought that since the bank had no record of a payment to come out on the 10th I did not need to do anything. I am very disappointed with the online service.