It is illegal for anyone other than the owner of a mailbox, or the postal carrier, to put anything in a mailbox.

If you have flyers that you want to distribute, do it the honest way. Pay for postage on them, and send them through the mail.

Do not put them in or on other people's mailboxes; that's a federal offense.

Do not attach them to people's doors, cars, or other property; that's trespassing, littering, and vandalism.

Legitimate advertising costs money. Don't look for ways to steal it. Nobody wants to do business with a thief; any more than with a trespasser, a litterer, or a vandal.