Charles Ellicott Commentary Esther 9:29

"Then Esther the queen, the daughter of Abihail, and Mordecai the Jew, wrote with all authority to confirm this second letter of Purim." — Esther 9:29 (ASV)

This second letter. —It seems to us that the first letter must be that extracted from the king by Esther (Esther 8:8), and consequently this “second letter” is Mordecai’s (Esther 9:20), which is now confirmed in a more authoritative way.