Did you ask to talk to the Postmaster General? I would go down and show them the slip and ask him/her where it might have gone.They seem to find things faster.The poor clerks have no idea.They don't deliver them, I have found when I have spoken with the Postmaster it seems to appear.
I completely understand and it sucks! I've written my package off as it's gone to postal heaven, or where ever it is they go when they lose them,.