To get the Post Industrial achievement in Little Nightmares 2 first you need to acquire the Postal Service hat that can be found in Chapter 4 (The Pale City). Then when you reach the area where you enter a vent and have to jump on the blockage to continue downward you can find a suitcase you can open just below where you clear the blockage. Inside the suitcase is a package that must carry to the far left side of the screen and a hand will reach through the letterbox that is there and take the package from you. Here is my own video guide that shows how to get the Post Industrial achievement along with all of the other achievement/collectibles Little Nightmares 2 has to offer. The locations of the Postal Service hat is at around the 20:04 mark during the video and the package location is at around the 20:59 mark during the video.