mailing first class refers to sending mail through the United States Postal Service’s First-Class Mail service, which is one of the most common and affordable methods for sending letters, postcards, and small parcels. First-Class Mail offers delivery in 1 to 3 business days, depending on the destination, and includes tracking and delivery confirmation for eligible items. It is an economical choice for individuals and businesses who need reliable, fast mail delivery within the U.S. without breaking the bank.