The letters strewn all over the floor will readable by players, and the letters’ content will be written by the players. Those selected will all receive credit. The development team hopes to encourage those who send their submissions to create and express “distress, horror, or bravery” to go with the zombie apocalypse theme. Imagine you are a survivor. Who do you write to? What do you say? The team wants writers to place themselves in that situation and then go for it.

Producer Tymon Smektala explains:

If you are interested in getting involved, the rules are here. 

The release date for the expansion has not been announced.

Dying Light devs want player written letters in post office themed expansion   Dying Light - 7