A flat can be a brochure, a catalog, a postcard or just about any allowable size mailing piece. The difference is flats are larger than a letter-size mailing piece and cost more to mail too. It may or may not be worth the postage cost but you can mail a piece up to 15″ in length and up to 12″ in height. First-classfFlats are very expensive to mail. One reason is that flats are so large they typically weigh more than one ounce. And with first-class mail you pay for each ounce the mailing package weighs. First-class flats start out at $0.92 per piece and can be mailed for as low as $0.406 per piece; but that’s only for the first ounce. Standard flats are also more expensive but you get 3.3 ounces in weight before the price goes up. Standard flats start out at $0.50 per piece and go as low as $0.201 per piece, but very few mailing projects qualify for that low rate.
Dimensions: Minimum: more than 11-1/2 inches long, or more than 6-1/8 inches high, or more than 1/4 inch thick. Maximum: 15 inches long, 12 inches high, and 3/4 inch thick. The length of a flat-size mailpiece is the longest dimension. The height is the dimension perpendicular to thelength. Shape: rectangular, with four square corners, or with finished corners that do not excced a radius of 0.125 inch (1/8 inch). Other specifications may apply