Bad customer service paying for something that I never signed for!!

I'm very disappointed, shocked and angry I bought clothes on credit at Mr price for R640. I went to make a payment today and I was told I owe R1390 due to some policy I was not even aware off that I don't even need I have ****** and unfairly treated because my financial situation is not in my favour how can they treat their customers this way. Kindly assist me hello Peter!