In order to qualify for postcard postage rate, the card must fall within these dimensions:
We prefer a size of 4¼″ x 5½″ because it fits four pieces to a 8½″ x 11″ sheet of paper. Sizes larger than the maximum size will cost $0.44 to mail instead of $0.28.
The maximum size for a letter in an envelope 1 ounce in weight or less is 6⅛″ x 11½″ x ¼″ thick. Anything over that size will incur a surcharge on top of the $0.44 first class rate, but only if it’s 1 ounce or less. Letters over 1 ounce in weight will require additional postage regardless of size.
Any item larger than 6⅛″ x 11½″ x ¼″ thick and smaller than 12″ x 15″ x ¾″ thick is considered a flat. It may weigh from 3½ ounces to 13 ounces; the rates start at $0.88 for 1 ounce. It is recommended that a sample of any mail piece be weighed for an accurate price.
When designing your self-mailer, it is important to remember that the U.S. Postal Service prefers to have the fold at the bottom and the opening at the top. A self-mailer must be tabbed at the top to prevent it from opening while the postal service is handling it. To allow the mailer to pass through the postal sorting equipment, the minimum paper weight for a self-mailer is 60-lb.