Albert Barnes 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 of Purim - Mordecai’s first letter (Esther 9:20) was to some extent tentative, a recommendation. The Jews generally having accepted the recommendation (Esther 9:23), (Esther 9:27), he and Esther now wrote a second letter which was mandatory.