In this letter, Daniel tells his daughter that the people in his cell are very kind, and entertain him with jokes. He explains that he has an “angel” who is delivering his letters to Marie-Laure. Finally, he tells Marie-Laure that he’ll be able to receive packages. He asks Marie-Laure to send him something, and concludes that he is “incredibly safe.”
As Daniel sends more letters, it becomes increasingly clear that he’s lying to Marie-Laure to keep her from worrying too much. He keeps repeating that he’s safe, happy, etc., but this seems unlikely—otherwise, he’d have a better way of delivering letters (and wouldn’t be so grateful to send a simple letter to his daughter).