The seller has two choices when the item is refused or not picked up.
When he gets it back (and returned mail is a very low priority for post offices) he can refund you your original payment LESS the shipping cost and any return shipping the PO charges him.
Or- he can contact you and ask if you still want and require you to pay both return shipping costs (back to him then back to you).
If you are going away, you can buy Hold service from the post office, but this usually does not include packages because most POs just don't have room.
You could also buy a Change of Address and have all your mail sent to a trusted family member or friend.