I was wondering how to complete this challenge, and I saw from ChrisHateZ answer to a similar question that you need to:
"…transport this postcard over your whole journey in the cave to return it right at the end.
Since this method is obviously tedious, I'm wondering if it would not also be possible to:
Leave the postcard on the ground somewhere in the lobby, then return it to the rack right at the end of the game.
Does my proposed alternative method work? Essentially the question boils down to whether the achievement is tied to the final object interaction, or whether it is tied to a process of steps that have to be taken in a specific order. I'm also interested if the steps for the "Shoplifting" achievement must be completed on the same run as the "Remorse" achievement.
Given the very long period of time it takes to get this achievement, it would be very helpful to me if someone could confirm the exact requirements before I attempt it.
Best Answer
To get the achievement, you just have to make sure you have the postcard in your possession at the end of the game. You can set the postcard down at any time, as long as you pick it back up before you leave an area. (Otherwise some puzzles would be impossible because you need all three characters to hold a different item at the same time.)
However, for the trick you suggest (leaving it in the lobby at the beginning of the game): my guess would be that it would not be there when you return to the gift shop at the end of the game. I am going to do my second play-through soon, so I will try it and let you know.
It is not really a hard achievement if you still plan to play through the game again with other characters. It would just be a pain in the neck if you have already played through the game with all the characters because you would have to do another play-through just to get it the trophy.
Update: I tried your suggestion on my second play-through. I left the postcard on the floor of the lobby at the beginning of the game. At the end of the game it was not there still. I think you have to lug it with you.