At 5pm the shutters go down.
And fair enough, too; post office staff are people, too, with their own lives to lead; It's bad enough they've to put up the stench until 5, let alone past 5. It's not like you're forced to wait until the last day to file. Why are you even filing in paper, anyway?
"Poor planning on your part does not necessitate an emergency on mine.”
-- Bob Carter