Interesting fact I came across while making our homemade new years card. The maximum thickness for the special, cheaper postcard rate is 0.016 inch (maximum size is 4.25in x 6in).
But the nice thick Enhanced Matte posterboard (24”x30” sheet) is 0.047 inches (1.2 millimeters) according to: http://pe.usps.com/text/qsg300/Q201.htm
I also see there are limits on stiffness so probably this material will not qualify for the postcard rate.
Examples of a nonmachinable letter include:
- It has an aspect ratio (length divided by height) of less than 1.3 or more than 2.5 (a square envelope has an aspect ratio of 1, making it nonmachinable).
- It is more than 4-1/4 inches high or more than 6 inches long and is less than 0.009 inch thick.
- It has clasps, strings, buttons, or similar closure devices.
- It is too rigid.
- It has a delivery address parallel to the shorter side of the mailpiece.
- It contains items such as pens, pencils, or keys that create an uneven thickness.
- Letters with nonpaper surfaces, other than envelope windows or attachments that are allowed under eligibility standards by class of mail.