How many stamps do you put on a manila envelope?

The United States Postal Service classifies a manila 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 for 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 Priority Mail flat rate envelope is an additional choice for mailing heavy documents that require faster delivery.