The United States Postal Service classifies a 9-inch by 12-inch envelope as a large envelope or a flat. Mailing such an envelope requires two first-class postage stamps for the first ounce and additional postage for each additional ounce. Large envelopes that are thicker than 3/4 inch require package-rate postage.
Any envelope larger than the maximum dimensions of a U.S. postal letter (6 1/8 inches high, 11 1/2 inches long and 1/4 inch thick) is classified as a “large envelope” and requires extra postage. Maximum dimensions for large envelopes or flats are 15 inches long by 12 inches wide by 3/4 inch thick. A flat-rate envelope is another choice for mailing heavy documents that require faster delivery.
In addition to the size and weight of the envelope, keep in mind that the USPS requires that large envelopes and other flat mail be sortable by machine and flexible. If shipping a non-flexible piece of mail, an extra surcharge will be added to the shipping price.