The USPS pricing structure takes into consideration the cost of transportation based on size, shape and weight, and the different costs of processing mail. For example, a letter that is machinable costs less than a flat or a parcel that has to be hand-processed.
Shape is determined by width, length, thickness, and weight. Shapes that determine First-Class mail pieces are postcards, letters, large envelopes (flats), and packages (parcels). If a letter exceeds 1/4″ thickness and/or 3 1/2 ounces, it is charged as a flat. For First-Class packages, length + girth combined cannot exceed 108 inches. Length is the longest side of the parcel. Girth is the thickest point.
Non-Machinable Letters
Non-Machinable Letters with the following characteristics are subject to package (parcel) pricing:
Examples of non-machinable letters are:
A non-machinable letter weighing 3.5 ounces or less will be charged the appropriate postage rate plus a surcharge of $0.20.
A large envelope that meets the weight and measurement criteria, but is determined to be non-machinable, will be charged as a package.