A nonmachinable mailpiece is a mailpiece that must be sorted outside of the standard, automated mail process. Because it is more expensive to processes these mailpieces, a surcharge applies to them.

What makes a letter non-machinable?

Examples of a non-machinable letter:

A letter that has an aspect ratio (length divided by height) of less than 1⅓ or more than 2½ (a square envelope has an aspect ratio of 1, making it non-machinable). ... A letter that contains items such as pens, pencils, or keys that create an uneven thickness.

Why does my mail say non-machinable?

If any of those criteria are not met, your envelope is “non-machinable” — meaning it cannot be processed by a USPS machine — and you should add extra postage. ... Nelson's site not only explains current US postal rates, but also helps you to figure out what “standard sizes” of envelopes actually are.