A First-Class Mail ® Forever stamp costs $0.73 and goes in the upper right corner of the envelope. (You can also use any combination of stamps that adds up to $0.73.) If your letter is heavier or bigger, or if you want to add insurance or extra services like Certified Mail ® service, you'll pay more. A standard postcard stamp costs $0.56.

Postal service in the United States began with the delivery of stampless letters whose cost was borne by the receiving person, later encompassed pre-paid letters carried by private mail carriers and provisional post offices, and culminated in a system of universal prepayment that required all letters to bear nationally issued adhesive postage stamps.