The most that a letter can weigh and still be shipped using the United States Postal Service's First-Class service is 3.5 ounces. Letters weighing more may be subject to additional postage charges. Letters that do not bend, have clasps or are lumpy may also incur a nonmachinable surcharge.
The USPS classifies mail as a letter if it is in an envelope no larger than 11.5 inches long, 6.125 inches high and 0.25 inches wide. The minimum acceptable letter size is 5 inches long, 3.5 inches high and 0.007 inches thick. Letters that have the address on the shorter side of the envelope or that are larger than 5-by-5 square inches are subject to an additional postage surcharge.