Later in the story the main characters go looking for the bad guys at a gun show. As DeNiro and Murphy enter the event next to a howitzer immediately to their right is two mail carriers ( Postal workers) in blue uniform with blue pith helmets checking out German machine guns. Obvious reference to incidence of post office workers having gone "postal".
When Trey is following Preston he crouches behind Preston's car. The license plate is in shadows, but then is suddenly lit up like someone turned on a light.