Truworths seems unreasonable regarding the interest charged

I wish to make a complaint about Truworths in towards the end of March a pair of boots were bought for a friend of mine. The boots were approximately R375.00. Every month I have made payment of R100. which was more than asked for installment amount. Thus far an amount of R300 has been paid in total however on the Truworths system, currently they still want a R110. How does this work the amount of interest that is being charged is ludicrous in lieu of the fact I have paid more than what my installments on a 6 month account. This does not encourage me to buy much on my account.