Automation flats mailings are very different from automation letters and cards. If your mailpiece weighs more than 2.5 ounces, or is bigger than 6-1/8 inches high by 11-1/2 inches long by 1/4 inch thick, choose flats. The maximum size for a flat is 15 inches long by 12 inches high by 3/4 inches thick. If your mailpiece is bigger than that, Envelope Manager cannot presort it. If you are mailing a postcard whose size falls between 3-1/2 by 5 inches minimum to 4-1/4 by 6 inches maximum, you can send it either as a First-Class Mail card or a Standard letter. First-Class card rates are slightly cheaper, but subject you to the address update requirement. Most folded flyers qualify as Automation letters, provided they are sealed according to USPS regulations. Typically that means using tabs, wafer seals, or glue--no staples!