Exchange day light *******

I purchased items online that were on special online but when the arrived we needed to exchange some items. I checked online about exchange policy I found nothing, I called customer service and I was advised I was I can exchange the items in store at no extra cost and won't forfeit the online discount. I went to Mr Price at Boulders shopping center in Midrand to exchange the items and I was forced to pay and forfeit the discount I received online. I fought with the manager and explained I'm not supposed to pay I'm just exchanging the exact same items for a different size. She failed to do her job and handled the whole matter in an *********** manner