First-class mail is a type of mail often sent through the U.S. Postal Service. It includes postcards, letters, large envelopes and small packages that are under 13 ounces in weight. First-class mail is given the highest priority for delivery speed.
According to the United States Postal Service, rectangular-shaped envelopes are ideal for personal correspondence letters, bills and light merchandise. First-class mail is typically delivered within one to three business days. It is different from second-class mail (newspapers and magazines), third-class mail (bulky packages) and fourth-class mail (books and media). In the U.K., first-class letters are prioritized over second-class, at a higher delivery cost.