On a more serious note, I’ve actually camped out by a mailbox before.
Waited for the mailman to show up so that I could ask for my letter back.
He gave me a mock hard time at first, but gave me my letter back.
Not sure if that is their policy though. I must have just looked really pitiful.
But if it’s already been processed, I highly doubt you could get it back.
PS If they don’t wander in of their own accord,
try PMing Astrochuck (works for USPS)
or Galileogirl (her dad used to work for USPS).