A standard first-class mail stamp provides accurate postage for domestic letters weighing no more than 1 ounce. Various factors may influence the overall weight of a package, including the weight of the paper inside and the size of the envelope used for mailing.
All letters that weigh more than 1 ounce incur additional postage charges. A first-class letter may weigh no more than 3.5 ounces total. After crossing this threshold, a letter is considered a small parcel for mailing purposes. Furthermore, all first-class mail pieces must weigh less than 13 ounces to be eligible for these rates. Heavier packages may be sent via Priority Mail.