Confused about pricing model at Mr Price

I Bought a tracksuit and track to for my daughter a few days ago Monday she's ten years old got a 11-12 Tuesday I returned the pants for a size bigger. The set was R60 top R60 bottoms went I went to exchange the pants I had to pay R99,99 for the pants because I didn't return the top with the pants but my top clearly shows I bought both. Why should I exchange the top if it fits and nobody said there only R60 if I buy both which I did and I just came to exchange size. Why was I charged an extra 39,99. No one at the Norwood branch could assist me that's where I bought and exchanged.