Over charging customers

I was charged a club magazine I did not join. I paid off the balance and left the r49 that was charged. We called the customer service from the store to ask them to cancel and refund the r49 back to my account. The consultant I spoke to said he has done it. I went to the store to make a purchase after few days only to find that I owe them r120 something. They said I was charged interest of about r50 something and for sms. I don't understand why am I charged so much interest with the balance of r50 and still that was them making me to join something that I did not agree upon.