The USPS has changed the way they classify and charge for mail services. 

The biggest change is that the mail rate applied to your letter mail can change depending on thickness, contents and the envelope used. The following characteristics cause a letter to be nonmachinable and therefore charged an additional $.17:

• Has an aspect ratio (length divided by height) of less than 1.3 or more than 2.5
• Measures more than 4.25” high, more than 6” long, or is less than .009” thick
• Is rigid or contains odd-shaped contents such as a pen, loose keys, coins, or tokens that cause the thickness to be uneven
• Is polybagged, polywrapped, or enclosed in any plastic material.
• Has clasps, strings, buttons or similar closure devices
• Is a self-mailer with a folded edge perpendicular to the address and is not secured (e.g. tabbed)
• The delivery address is parallel to the shorter dimension of the mailpiece
• Booklet-type pieces with the bound edge (spine) along the shorter dimension of the piece or at the top, unless prepared according to 201.3.13.2

The following characteristics cause a flat to be nonmachinable and therefore metered at package rates:

• Is rigid
• Contains odd-shaped contents such as a pen, loose keys, coins, or tokens that cause the thickness to be uneven
• Has a corner radius greater than 1/8"
• Is polybagged, polywrapped, or enclosed in any plastic material.

If you send a flat mail piece with any of the above characteristics, it will be charged as a parcel.

Links:

Shape Based Mail Piece Decision Tree
Detailed USPS Rates and Fees Information
Postcard, Letter and Flat size requirements
Letter/flat size template
Delivery zone chart

If you have any questions, please email Diane Cummins at dcummins@palomar.edu or call at Extension 2754.