Max's final (on-screen) letter to Mary, forgiving her and pulling her out of her depression.
The final scene in the movie, with a bit of foreshadowing earlier. Max had ironed, pressed, and laminated all of Mary's letters, presumably to keep "in a special place". When Mary comes to visit, Max had died that morning while looking up at the ceiling. She looks around the apartment, seeing some of the gifts she'd sent before looking at the ceiling herself, which is covered with all of the letters she'd sent him over their 20+ year correspondence. Couples with tearjerker, but that's a devoted friend.
Max: You are my best friend. You are my only friend.