Mail of any kind for transport by the US Postal Service should be enclosed in an appropriate envelope or parcel and sealed. The types of enclosures determine the mailing classification.
Paperclips and staples are examples of non-mailables in envelopes.
The following are also classified as non-mailables and may be returned to sender:
Envelopes and Postcards if these pieces are less than 3 1/2" in height or 5" in length, must be 110lb card stock or heavier
It is recommended that when mailing questionable items, the mailer should call Mail Services for assistance.
Window Envelope Enclosures
Enclosures that are designed so that the address appears in a window must not be stapled to prevent slippage of the address from window view. If the addressed enclosure does not properly fit the window, use an envelope without a window.