I got this right off the USPS.com site:
First-Class envelopes weighing 1 ounce or less require an additional $0.12 nonmachinable surcharge if any one of the following apply:
It is a square letter
It contains very rigid items such as wood or metal
It has clasps, string, buttons, or similar closure devices
It has an address parallel to the shorter dimension of the letter
It contains items such as pens that cause the surface to be uneven
The length divided by height is less than 1.3 or more than 2.5
If unsure, you could always call your local postoffice, but depending upon who you talk to, you may get mixed answers. If in doubt, send one thru to yourself, and see what happens!
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