Update: I believe this is still active, though it doesn't seem to work on mobile.
Walmart has restocked their free baby welcome boxes! In order to access the signup form, let the page completely load. Then look for the pink “Welcome Box” text near the top left. It looks like this:
After clicking that, a new page should load with a signup form. If that page doesn't load, please let us know in the comments, as that means the free sample box has probably expired for now.