Mr Price makes loyal customers pay for the "sins" of other customers

I wish to make a complaint against Mr Price. I bought 1 item last month with was a top for R60 when I come to pay my account I am told that the installment I must pay is R45. This is absolutely ludicrous as it is a 6 month account and I packing R45 due out of R60. How can this be acceptable? I thought the purpose of having a 6 month account was to pay the items off in 6 months not one! Clearly we as loyal customer are being punished for other customers in huge debt.