Federal law makes it illegal to place flyers, ads, or any other unstamped material in or on a residential mailbox. The prohibition comes from 18 U. § 1725, which applies to every mailbox approved by the U. Postal Service, and violations can carry fines up to $5,000 per. Putting flyers in mailboxes is illegal, but there are legal, effective ways to distribute them. The. Is it illegal to put flyers on cars, doors, and mailboxes? If you did not stamp the flyer and send them through USPS, it is illegal to put flyers in mailboxes. Flyer distribution enables reaching many potential new customers. Let's start with the things you should always do to maintain proper mailbox etiquette and ensure smooth mail delivery.