Capitec Bank harrasing me.

I made arrangements with Capitec for them to deduct R1 500 on 20th of each month. They send me a message to say this month they will deduct earlier, they did on the 13th June..I called to ask why I was told it's a system error and that they will still deduct again on the 20th. I made sure the is no money cuase they deducted on the 13th June instead of the 20th.. Now I get a letter of demand. I don't understand why because they deducted for June. Capitec bank is so disgusting and they harrasing me. Please Help.