For context: in Reisa's first 3D video (original | subs), she prepares two "letters of challenge", one for Kazusa and the other for Sensei. The one meant for Sensei, as she explains, is just a normal letter of gratitude, with things she couldn't say in person. But it turns out Reisa gives Sensei the wrong letter, and the one intended for Sensei is already on its way to Kazusa.