TFG is a ****

I have been paying my account every month on the 25th. Last month when I asked for the amount i owe, i was surprised when the cashier said it’s R 7000. I went to enquire about the issue, they only explained that day i have to pay on the 1st of every months hence the amount increased to that extent.

Can someone help me because it’s gonna be difficult to pay such amount. Why didn’t they inform me first on which day to pay, i would have cancelled it immediately because i get paid on the 25th. I wasn’t aware i have to pay on the first of every month.