You're exchanging shoes that you've been wearing already?
Sorry but this does not sit well with me if this is the case. You bought the wrong size and had the choice to return them while they were still new and unused. It is not the store's fault and/or error that you chose to wear them on and off for 3 weeks.
I apologize in advance if I am wrong and misread your posts.
They were still practically brand new. And for the price I paid I think I have the right to exchange them if I need to.