Amazon doesn't need to own the last mile. They just need to ensure whoever does meets their standards. USPS has rules for what size packages they will deliver. Amazon either needs to get USPS to follow their rules and deliver that package, or they need to read the rules and ship via someone who can deliver. (I don't know the rules or package in question so I cannot say who is at fault). Of course Amazon might be too small a player in the eyes of USPS, but there are other choices for delivery if USPS doesn't meet their needs.