what is the max weight allowed in a standard first class letter
Maximum size: 6 1/8 inches tall x 11 1/2 inches long and 1/4 inches thick. Maximum weight: 3.5 oz. Aspect ratio: Length divided by height: Ratio must be between 1.3 and 2.5 or the letter will be subject to a non-machinable surcharge. See step 3 for details.