Normally not have to send back thanks Letter But case of keep relations and special invent happen between You and the person each case is not same but start with / ご丁寧な返書を賜りありがとうごさいます 。。。 after actual feeling and why impress or thank to others Then end up business / 貴社(あなた) のご厚情に感謝し末長いつつきあいのほど よろしくおねがいします/ private / 本当にありがとう、またよろしくおねがいします this is just a formal example Just sent thanks card is fine and they happy to get message. No need long letter Just express your thank is enough.