Worst customer service and ********* practices

If I could give 0 stars, I would. Last year we purchased a bed and microwave on credit from Lewis stores Wellington. Upon delivery after a very long wait, I noticed that the box of the microwave has already been opened before it landed on my doorstep. I contacted the store, to come fetch it, because I am not accepting something that has been opened or is a demo. The store collected the microwave, and then informed us that there is no more of those microwaves left, whereby I asked them to take the microwave off the account. After months, and a lot of terrible customer service from Head office and the previous store manager and team, the cost of the microwave was taken off, but the monthly repayment remained the same. My question is, why am I still paying for interest on a microwave that I do not have??? This is daylight *******, and it is ********* to make someone pay interest on something they do not have.