Any letter smaller than the minimum size is not mailable. Letters that exceed the maximum size in height, weight or thickness, are subject to a surcharge since the envelope cannot be processed through post office machines because of its size or thickness.
If an envelope is over 6 1/8" tall or 11 1/2" long, the price goes to a flat rate of 80 cents.
While most of the changes have size determining price, weight is still a factor. Any letter weighing 1 oz. or less is 41 cents. Anything over 1 oz. starts at 58 cents and goes up from there. Anything weighing 1 lb. or more is considered Priority Mail and is priced accordingly.
If a letter is thicker than 1/4", it will be charged a flat rate of 80 cents. To avoid this flat rate fee, try putting the contents of the letter in a larger envelope, and folding once instead of twice. This will help reduce the thickness and the price of your letter from 80 cents to 41 cents if under 1 oz. and 58 cents if 2 oz. (envelope must be a standard letter size for these prices).
If the weight exceeds 1 oz., price of the large envelope will increase 17 cents per ounce. If a large envelopes is rigid, nonrectangular or does not have a uniform thickness, customers will pay package rates.
5" x 3 1/2"
6" x 4 1/4"
0.007" to 0.016"
Any postcard smaller than the minimum size is not mailable. Postcards that exceed the maximum size are charged the First Class letter rate of 41 cents.
However, if a postcard is larger than 6 1/8" x 11 1/2" and the thickness exceeds 1/4", the price will be 58 cents.
Packages are priced according to weight, length and girth. Length is the longest side of the package and girth is the measurement around the thickest part. To see package prices, visit the United States Postal Service's rates and fees page online.