The envelope must be strong enough to protect and retain its contents. The use of oversized or overstuffed envelopes can lead to damaged or lost contents.
Window Envelope Enclosure
Enclosures can be designed so the address appears in a window envelope. Do not staple 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.
All mail must be sealed in an appropriate envelope or parcel. When a periodical or other publication is mailed with non-incidental First Class mail or any standard mail enclosure, the mark "First Class Mail Enclosed" or "Standard Mail Enclosed" must be placed, respectively, on or in the host publication. The type of enclosures determines the mailing classification.
First Class, Third Class and International Letter Class mail weighing one ounce or less is classified as non-standard if it exceeds any of the following size limits:
- Length: 11 1/2"
- Height: 6 1/8"
- Thickness: 1/4"
There is an additional charge on each piece of mail that is found to be non-standard. In addition, mail that is less than 3 1/2" in height, 5" in length, or the thickness of a postcard are non-mailable in the U.S. and are may be returned to sender.