Yeah, do NOT put anything directly in to any one else's mail box. It is a violation of US law (18 USC 1725) to put anything in or on a mailbox that does not have properly-paid postage. (Funnily enough, you do not have to actually SEND it through the USPS, it just has to have proper postage...)
When I was first starting my business, my wife and I went out distributing fliers in our neighborhood. My wife decided it was easier to tape them to mailboxes than to go up and tape them on doors. About two days later, I got a nastygram from the local US Postal Inspector. An apology was sufficient, no fine since it was the first time without a large distribution, but still...