A handy bit of feedback on the back of a letter I sent in a while back - if you put it in the envelope wrong, you get a nice message through the window:
But I wonder: Will people check for the presence of text (and call it good), or will they read “Wrong Way” and fix the error?
Wait, how can you put it in the envelope wrong? Is…it when you don’t show the address through the window?
Exactly - in his case, through the window it says “Wrong Way.”