Cant exchange online purchases instore

I bought items online during the 3 for 2 promotion.

I wanted to exchange one item and read the document which came with the order, it stated clearly that i should take tgat said document and the initial invoice emailed to me which i did, to my horrow I was told I if exchange (same item) I will have to pay. I would understand if i was told to bring all the items in order for tgem to assist with an exchange not that the rules arent the same when you buy online. In store yes you can just exchange however when you bought online we you pay the difference. Why do you havean onl8ne store when it's just a tideous process just to exchabge i would understand if i was asking for a regund, i just wanted a bigger size. Im so disappointed

I went to Wonderboom and was assisted by Zandile