Extra charges

It seems like the more I pay the Woolworths account per month the more it increases. My installment will be R89.46 then I pay R100.00 but still the balance doesn't decrease because there are other charge like monthly account fees, Interest Charges. what are the interests for if I pay my account on time every month. Balance for February R884.14 after making R100 payment, March balance is R828.60. which means i only pay R55.54.