laser-PIR sensors
Seem to be mixing things up here.
PIR stands for Passive Infra Red - nothing to do with lasers. It is meant for detecting the IR from living things that they emit due to their temperature. You can not detect mail with a PIR.
and it's not worth the effort of getting the junk mail till there's also real mail in the box.
In the UK the mail man can't delver junk mail unless there is some real mail to delver to you. However that only means mail with your address on it, it could contain junk as well.