Il m’envoie is correct. Il envoie moi is not. With an affirmative imperative sentence, moi is placed after the verb such as envoie-moi un cadeau
Tu vas me manquer means I am going to miss you/ you are going to be missing from me. It’s a specific construction with the verb se manquer. In French, the person missed is the subject of the sentence, whereas in English, the person missed is the object.
Tu vas me has no meaning by itself. Tu vas means you are going.