Thanks Grumpy_Mike for your feedback and input.
I was thinking about using two laser-PIR sensors (one top-to-bottom, and one side-to-side); however, there might be a postcard in the front half of the box or a postcard might be in the back half of the box. I'm just thinking -- maybe I can put the laser-PIR sensors right in the front of the box -- so the beam would be broken when the mail goes into the box, but the beam would not be broken while the mail is sitting in the box. So, I would just have to register that the beam had been broken (when the mail was placed in the box) and not, necessarily, have to detect whether there was any mail sitting in the box (after it was put into the box).