Thanks for the input -- this is giving me some ideas. The mailbox is in an office suite with many businesses. There is a large door which gives the secretary access to forty (?) mailboxes. Once the large door is closed, each mailbox has its own little door -- thus, the secretary does not open the little door; and opening the large door (which opens all of the little doors) does not, necessarily, indicate that there is any mail in my particular mailbox. Taking that into consideration, I could use the opening of the large door (which, also, opens my little door) to trigger a reed switch (or, maybe, a Hall Effect switch). The reed switch could, then, trigger the laser in the opening of my particular mailbox to activate and, thus, detect the passage of mail into my box. The laser would, then, automatically turn off after a period of time or until the Reed switch or Hall Effect switch detects that the large door is closed. So, everything would only be on for a short period of time. I understand the whole thing could be put into sleep mode; I would only need to activate the Hall Effect and/or reed switch during business hours. If I use a reed switch, I wouldn't even need to put everything into a sleep mode; everything would just turn on when the reed switch closes the circuit. I'm just thinking outloud here; your inputs are very helpful! Do you think that would work???
Apparently, instead of a laser and a PIR sensor, I would need a laser and a photocell (thanks for that clarification).