Mail of any kind for transport by the U.S. Postal Service should be enclosed in an appropriate envelope or parcel and sealed. The types of enclosures determine the mailing classification.
(a) Rigid and Odd-Shaped Items
Rigid items (e.g., pens, pencils, keys, bottle caps) are prohibited within mail pieces. Reasonably flexible items (e.g., credit cards) are permitted. These are subject to section 201.3.12, Flexibility Standards for Automation Letters, odd-shaped items (e.g., coins and tokens) are permitted if firmly affixed to and wrapped within the contents of the mail piece and envelope to streamline the shape of the mail piece for automated processing.
Letters that are non-machinable or contain rigid items are subject to a non-machinable surcharge.
(b) Non-machinable Criteria for letter size mail
Please use the following link to get the latest information: http://pe.usps.gov/text/dmm300/201.htm#wp1042571 Refer to section 201.2.1
Window Envelope Enclosures
If a window envelope is used for a letter or flat size mail piece, the clearance between any printing or window edge must be at least 0.125 (1/8) inch. The top and bottom window edges must be at least 0.040 (1/25) inch for any address side. These clearances must be maintained during the insert's range of movement in the envelope. If the addressed enclosure does not properly fit the window, use an envelope without a window. Please do not staple mail into envelopes. Staples can jam Postal equipment and destroy your envelope.