For the U.S. Postal Service to charge the lower postcard rates, the size of a postcard must meet specific requirements. First, it must be rectangular in shape. Different sizes are okay; different shapes are not. Then the postcard must be at least 3.5” x 5 inches and 0.007” thick. ...
First, you must ensure your direct mail postcards meet the sizes and quantities required by the USPS® to be eligible for either Standard letter rates or First-Class® presort postcard rates. Then it’s a matter of choosing whether to do the presort work yourself or engage a presorting ...