Make sure you return the defective item to the seller with delivery confirmation. Then if the seller doesn't refund you after receiving the item,you can open an item not as described dispute through PayPal. Add the delivery confirmation number to the dispute and you should get a full refund of your original payment,including the original shipping fee as soon as PayPal verifies that the seller has indeed received the returned item.
Remember,you only have 45 days from the time you paid to open the PayPal dispute,don't let this time limit expire.