In the first location of the DLC, the pretty suburban street, there are mailboxes in front of houses. I counted seven. The last one was in front of the drive-in movie theater. Smash them as you are progressing though the story.
(if you don't know how to access the dlc, you will need a LIFTER HOOK - so progress through the main campaign enough to get it, then head to Port Nixon, down the exo lift in the Iron Maus store and in the open area near water you will see an orange boat. It will take you to a small islet from which you need to use the lifter hook)