High interest rates

I have an account with them and every time when I make purchases I also choose 6 months options because I was told that it interest free now all of the sudden my instalment has increased so much and last time I phoned consultant told about some stupid excuse that it depends on my purchases if I buy more that instalment increase if I haven't finished paying for previous items which I didn't understand, my Credit limit is only R3100 and I received a message that my instalment was R660 then I went to purchase few items within the limit mentioned then three or four days later I got an sms that my instalment this month R800?????? What is your interest that you charging people here? on a limit of R3100??? guess what I'm going to pay this R800 and reduce this limit until I close this account because this is a *******, my sister told me this morning that she paid last month and closed hers because of the same thing and I will never recommend Truworths to anyone!!!!!!