Is MR PRICE over charging on their interest Rate and Installment Repayments?

I have been trying to get a clear statement from MR Price so I may Repay the installment amount on my account, I have queried for a full breakdown on why my installment amount is higher even after I factor in the 18.75% interest rate on this clothing account. I requested a breakdown via mail to absolutely no response, I also called, I clicked the accounts option and asked this question, I first called on the 26/07/22 and I was told please hold as I put you through to accounts however when I called and the options were given to me, I choose the accounts option so why are you putting me through a department I already am in, I called again on the 27/07/22 and got told the very same thing so I believe they train the staff not to answer such queries as there are some activities/practices that are highly questionable. I just need to know why the rate of my repayment is as high as it is which should be a simple explanation however Mr Price is showing that it is not. Please may I get a full breakdown of my repayment costs Mr Price especially since you are meant to be practicing in good faith.