5x7 Postcard Postage
When sending a postcard, the USPS recommends that you use a rectangular, 0.007-inch-thick envelope. The envelope must be three-1/2 inches high by five inches wide and no larger than six-1/2 inches long by one-fourth of an inch thick. The maximum dimensions for 5x7 postcards are three-1/2 inches high by five inches by one-fourth of an inch, and the maximum weight of the mailpiece is two ounces. However, if your card is bigger than these specifications, you must pay for a flat-rate letter. The dimensions of a postcard are also important. A 5"x7-inch postcard is considered a standard-sized letter, but a 5x7-inch-thick postcard is not considered a standard-size postcard. A 6x9-inch-thick postcard is a First-class letter and will be charged the same postage rates as a 4"x6-inch-thick letter. Unless the card is oversized, the USPS considers a 5x7-inch postcard to be a First-Class letter. While it costs less than priority mail, it takes much longer to reach its destination. ...