Know Before You Mail
All pieces that are 1/4″ thick or less are nonmailable unless they are rectangular, at least 3-1/2″ high, 5″ long (domestic mail) or 5-1/2″ long (international mail), and .007″ thick.
Note: To determine nonstandard or nonmailable items, the length is the dimension that is parallel to the address when read; the height is perpendicular to the length.