Flat, flexible envelopes (Letters and Flats) require significantly less postage than rigid envelopes or boxes (Packages).
Letters and flats are flexible and flat, and are distinguished from packages by their combined flexibility and uniform thickness. Rigid or lumpy envelopes are considered ‘packages.’ Flat, flexible envelopes marked DO NOT BEND may also be considered packages.
Letters and flats are distinguished from one another by their dimensions:
Letter envelopes must be rectangular and made of paper, and can be a maximum of 11-1/2″ (292mm) long x 6-1/8″ (155mm) high, with a uniform thickness of up to 1/4″ (6mm). Flats (large envelopes) can be a maximum of 15″ (381mm) long and 12″ (305mm) high, with a uniform thickness of up to 3/4″ (19mm).
The maximum weight for Letters is 3.5 oz (100g); the maximum weight for Flats is 13 oz (368g).