Which optical sensor for detecting moving paper?
In my youth I had built something that depending on your mailbox physical dimensions may or may not work.

Essentially it was a thin metal sheet that was fixed inside the box at one end. It was thick enough to hold itself with a bit bending, but thin enough to be bent even more by the lightest envelopes.

You dropped one envelope in there, it bends down and makes contact, an led was lit up. 

No current consumption while no mail inside.