First up, what's the reason for the exchange/refund?
If you got sent the wrong item or a faulty item, this article should help.
Each store on Storenvy is independent and sets their own returns policy. All sales are final unless stated otherwise in a store's FAQ or Returns policy. If your order turned out to be the wrong size or you just don't like it as much as you thought you would please refer to the store's FAQ or Store Policies section on a product page to see what their returns policy is. Some stores will happily accept exchanges or refunds, no matter the reason. Other stores may only accept exchanges or offer refunds if certain requirements are met. You can contact the store directly from the order history page in your account settings or from the store's contact page.