ArtColter 
"Dear Spike, please tell Twilight to reach out to Shining Armor. I am to understand that he has something important to tell her, but it is really something that should be done face-to-face, and not via message. Best regards, Princess Celestia."
This would be Shining telling Twilight about his engagement to Cadance. Shining assumes that his sister received the letter, but was simply too busy to reach out to him. This is why Twilight was shocked by the wedding announcement, as she was not aware that Shining was engaged, and why Shining simply rolled his eyes at her exasperation with not being told, as she had the chance to hear it from him in the first place.
Admittedly, there are holes in this little headcanon, but it's something.
EDIT: Apparently, this is a common fan theory. In my defense, I did not join the fandom until after Season 3, so this possibility did not dawn on me when I watched the episodes the first time around.
This would be Shining telling Twilight about his engagement to Cadance. Shining assumes that his sister received the letter, but was simply too busy to reach out to him. This is why Twilight was shocked by the wedding announcement, as she was not aware that Shining was engaged, and why Shining simply rolled his eyes at her exasperation with not being told, as she had the chance to hear it from him in the first place.
Admittedly, there are holes in this little headcanon, but it's something.
EDIT: Apparently, this is a common fan theory. In my defense, I did not join the fandom until after Season 3, so this possibility did not dawn on me when I watched the episodes the first time around.