Dissatisfied Customer

I am dissatisfied with Mr Price ,I bought clothes online for my son ,and upon getting the clothes I noticed that they were big and I went to store MR PRICE SOUTHGATE to exchange and to my surprise I was asked to pay the difference on the clothes because I got a discount online ,why is that i had to pay the difference on the same items ,and to top it off the lady was like we should just refund you because we always tell customers that