Sick to the back teeth of take-away pizza flyers, UPVC double glazing folders, born-again carpet cleaner's cards etc etc?
Cut a grove with a Dremel on the inside bottom edge of the box flap. Stick in a 2P piece. Enjoy a litter-free doormat.
NB. this does rely on your postie delivering at the same time every day (preferably first thing in the morning), so you can take it out beforehand, and still get legit post (popping it back in before you go off to work). BP
Mad maybe. Happy, definitely. Genius, most certainly.