Which category includes items such as thick envelopes and large flat mail?

The category that includes items such as thick envelopes and large flat mail is indeed Flats. In the context of the USPS, Flats are defined as mail that is larger than a letter but not as heavy or bulky as a package. This category accommodates items like brochures, newsletters, and other similar mail pieces that are often too large to be classified as letters but do not meet the dimension or weight requirements to be considered packages.

Understanding the classification of mail is essential for proper handling and sorting within the postal system, as each category has specific regulations regarding size, weight, and postage. Flats typically can be up to 15 inches in length, 12 inches in height, and 3/4 of an inch in thickness, making them distinct from both letters, which are generally smaller and thinner, and packages, which are bulkier and heavier.