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My brother cracked the back of the phone and its completely cracked, but not the front, the back. How much is it to repair that? Including shipping and all that?

You could follow the first steps of this guide: Motorola Moto G6 Screen Replacement For repairs, you should check if there is a local store/repair which manages it. I’m not sure about the price, this is different regarding the stores / repair shops and country your in.