The size doesn't fit? No worries, you can create a new order to reserve the size and send the old one back with a filled out return form and the new order number. You don't have to pay for the new order, just select to pay with cash on delivery/ bank transfer and leave a note that it's for an exchange. If you don't have any of these payment options available, you can choose to pay with a credit card and close the window on the last step (when the only thing you can do is to enter your card info, which you won't do).
We recommend insuring the package before returning it. If the package is lost or damaged in transit, you could file a complaint with the carrier and get your money back.