Big misconception...
It is illegal to open someone's mailbox....you can hang fliers/pamphlets and what not from it *technically* if postage was paid....their domain is from the box to the ground (if on a pole).
I once put fliers in a neighborhood....all identical mailboxs and they had a little scroll on the pole, which I rolled up my pamphlet (christmas morning computer help) and stuck in.
I got a call and then a bill from the postmaster to pay (.10 cents I think each it came out to $10 or so)....for each I delivered.....I said 100 but the community had about 1000 houses....I can't find the letter, but it had the laws listed.