The United States Postal Service First-Class mail is delivered to the recipient in three days or less. First-Class mail is used by most businesses when sending out their billing information.
Postcards, letters, large envelopes and packages can be sent by First-Class mail but must not exceed 13 ounces. Postcards must be a minimum of 3 inches high and 5 inches long. Postcards that exceed 4 1/2 inches high and 6 inches long are charged the fee for a standard letter. The minimum size of a letter is 3 1/2 inches high and 5 inches long, and it must not exceed 6 1/8 inches high and 11 1/2 inches long or a thickness of 1/4 inch. The maximum weight for a letter is 3.5 ounces.