Dissatisfied customer

I’m writing this as a concern Woolworths customer with the dissatisfaction that I’m getting on their call Centre Agent WFS that I’m being billed for something that I don’t understand every month and to add I’m getting paid on the 20th of each Month but when I pay my account on the 20th I’m in arrears apparently they say their books closes on the 8th of each Month. Meaning that my account will be in arrears and I will be paying interest every month whereas when I opened the account I stipulated that I’m getting paid on the 20th.