Here is what it says on the USPS web site:

Nonmachinable Surcharge:
An additional $0.13 is required for items weighing 1 ounce or less with any of the following criteria:

A. Square letters.
B. The height exceeds 6-1/8 inches, or length exceeds 11-1/2 inches, or thickness

exceeds 1/4 inch.
C. The length divided by height is less than 1.3 or more than 2.5 (length is the dimension

parallel to the address).
D. It has clasps, strings, buttons, or similar closure devices.
E. It is too rigid or contains items such as pens that cause the thickness of the mailpiece

to be uneven.
F. It has an address parallel to the shorter dimension of the mailpiece.

It is on this page......

http://www.usps.com/consumers/domestic.htm#ns