To return a package to a merchant, contact them directly to obtain an RMA (Return Merchandize Authorization) with prepaid postage and indicate that it's for PICK UP. If the RMA doesn't include prepaid postage, you will have to get a prepaid shipping label from any courier that operates within the country of the warehouse origin. Same process if you are also sending the item/package to a different address.
NOTE: Please ensure that the return method selected is PICK-UP