Since game design is a business, I decided to see what's really selling well for the PS4.
I did this search a week ago, and at the time, out of the top 20 bestsellers on Amazon 10 had not even been released yet. By now some have been released. But others still have not. And yet others...
The Letters of the Lost are the focus of Far Cry 3's Dead Letters Trophy/Achievement. To unlock this piece of digital showmanship, you just have to find all 20 letters and collect them. This is a lot simpler than it seems, especially when you consider that the collectible maps are available for purchase at the general stores.
If you're super cheap or some kind of horder, you can use the video above to find the Letters of the Lost and unlock the Dead Letters Trophy/Achievement.