During the first level I found roughly 3 or so mailboxes which you can strike. When you do, they flash white until after so many hits, they smash open revealing either a bag, or a skull. As of yet, I've received no cash or rewards from doing this that I am aware of. Any clues, or is it just random?

Additional details - 3 years ago

It definitely seems like no one is quite sure. The idea it adds pigs in subspace would be reasonable, but every time you smash these objects, the sprite can be either a skull or a bag of money (not real money, just a sprite in the mailbox). I will leave this open until someone has a more solid answer...