Seven Steps to Navigating USPS Direct Mail Flat Requirements (piworld.com)
There are many times that a flat design element causes a mailing to go at a higher rate of postage. This can be frustrating as well as expensive. In order to help you stay away from potential issues here are some things to keep in mind as you are preparing a flat size direct mail campaign. USPS Flats: 1. Flat sized mail is between 126 x 11.51 to 12 x 15. 2. Paper stock must be a minimum of .009 thick. The maximum thickness is ¾ inch for the whole mailer. 3. No aspect ratio requirement. 4. Flats are required to have address blocks in the upper half of the short edge. 5. The fold or binding must be to the right of the mail panel. 6. No tabs are required. 7. If you use a poly bag/envelope the maximum extra space you can have inside the bag from the edge of the piece to the edge of the bag is 0.5.