Post Office - What is the PO Box used for?

The PO Box stack is used for when you want to show something on the page your Post Office stack is on, but not until the customers signing up click on the Sign Up button. You access this in the Post Office - Success Conent stack setting.

Choose Show PO Box in the On Success setting, and then you can add the PO Box stack on to your page. When you do, it will look like this:

Then you can most stacks into that area, and the users will never see it until the click the Sign Up button. Enjoy!