Mr Price - Why isn't there an option to change the size of an item

I don't understand why Mr price can't change an items size if I've mistakenly ordered the wrong size and the order hasn't even been sent yet but they still tell me its not possible. Is it that hard for Mr price group to allow that? Now I must go to the store to exchange my item. I'm really frustrated and I will never ever be ordering from Mr price again! How hard is it for these people to change one items size! Very incompetent.