You don't have to pay any postage fees. Packages for return must be packed in a way that is suitable for posting. This means that such packages must be packaged and labelled as if you were sending them by post. Use the box in which you received your order and tape the package back up.
Label the package with the return address of the online shop to which you would like to return the order. Some online shops send a return address label directly with their order or make it available for you to print it out on their website.