[WARNING: CONTAINS MAJOR SPOILERS FOR THE TRUE LIVES OF THE FABULOUS KILLJOYS COMICS]==
The Mailbox Shrine is an old world mail box decorated as an alter to the dead. Letters placed inside may reach loved ones in the afterlife, and masks placed inside supposedly let Phoenix Witch help guide the former wearer's soul to the afterlife. In the comic several masks of both Killjoys and Draculoids, including Party Poison's, are placed in the mailbox. The supposed witch who delivers these messages to the dead is seen in the comic, talking to the Girl who she spared from death.
The mailbox is seen in the "Na Na Na (Na Na Na Na Na Na Na Na Na)" music video where the Girl sends a letter to her dead mother and in the comic "The True Lives of the Fabulous Killjoys".
References : 'The True Lives of the Fabulous Killjoys' by Gerard Way and Shaun Simon, illustrated by Becky Cloonan.