Here are the USPS's characteristics for nonmachinable mail. If you send anything through USPS with these characteristics, they require an additional $.17 postage.
* It is a square letter (minimum size for square envelopes is 5 x 5 inches).
* It is too rigid - does not bend easily
* 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.
OK, Stampers ... basically all those ribbons, eyelets, brads, hardware, dimensional items that we put on our cards make it bumpy, creating an uneven surface. Therefore, USPS cannot process those envelopes through their machine. Relunctantly, I'll be adding the extra $.17 postage to my handstamped cards. Thought you might want to know!
More First Class Letter information
Minimum Height: 3-1/2", Minimum Length: 5"
Maximum Height: 6-1/8", Maximum Length: 11-1/2"
Minimum Thickness: 0.007", Maximum Thickness: 1/4"
A first class letter weighing over ONE ounce requires $.58 postage