You might find this helpful. Here is a link to the USPS pricing manual. Dimensions for letters, large envelopes and packages are on page 32.:
http://www.usps.com/prices/_pdf/not123.pdf
And from the USPS Domestic Mail Manual, here is a list of non-machinable characteristics that would be subject to the $.20 non-machinable surcharge:
1.2 Nonmachinable Criteria
A letter-size piece is nonmachinable if it has one or more of the following characteristics:
a. Has an aspect ratio (length divided by height) of less than 1.3 or more than 2.5.
b. Is polybagged, polywrapped, or enclosed in any plastic material.
c. Has clasps, strings, buttons, or similar closure devices.
d. Contains items such as pens, pencils, or loose keys or coins that cause the thickness of the mailpiece to be uneven.
e. Is too rigid (does not bend easily when subjected to a transport belt tension of 40 pounds around an 11-inch diameter turn).
f. For pieces more than 4-1/4 inches high or 6 inches long, the thickness is less than 0.009 inch.
g. Has a delivery address parallel to the shorter dimension of the mailpiece.
h. Is a self-mailer with a final folded edge perpendicular to the address if the piece is not folded and secured according to 201.3.13.1.
i. 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.
Linda