Calvina would be executed when Medis returns and it turns out Calvina didn't deliver the letter. But then again, I presume she's not planning to be in a vulnerable position by the time Medis returns. If she even believes Medis and Akira can return. Either Oswell pulls off a coup d'état or Calvina will leave Stromheim to join Oswell's side. So, in her plans there either is nobody around to punish her or Medis never returns and thus nobody will ever know Calvina was supposed to deliver a letter.
Still, either I was talking nonsense about Medis losing trust in Calvina or this is but a scheme by Medis. She could have given an identical copy of the letter to another person to deliver to the queen, with a postscript mentioning that if Calvina doesn't deliver a letter of similar content (minus the postscript), it means she has become a traitor and needs to be eliminated.
Edit: Although... I'm not sure the author has the skills to come up with even such a minor twist.