Are you allowed to put non letters in mailbox?

No, you are generally not allowed to put non-letters, such as packages or non-mail items, in a mailbox that is designated for postal delivery. Mailboxes are intended for letters and other mail items that can be sent through the postal system. Using a mailbox for unauthorized items may violate postal regulations and could result in fines or other penalties. Always check with your local postal service for specific guidelines.