tv box replacement

One of my TV boxes went bad and is dead. My Xfinity Store more than an hour away. Why can I not get a new one by mail and ship the faulty one back with a prepaid label? And since I rent this box, why should I have to pay anything to get a working one. I have chatted abou this but the information I get makes no sense at all and the person I chat with has a language problem so I don't undertstand what she means. Either I get a quick replacement without cost or I will skip Comcast and reconnect my Dish service!