Square envelopes are identified by the United States Post Office (USPS) as nonmachinable, meaning they cannot be sent through the typical postal reading machines and must be handled by human workers. As such, square envelopes require extra postage, called a nonmachinable surcharge. Nonmachinable items typically have one or more of the following characteristics:

• Rigid
• Square
• May jam postal equipment
• May be difficult to process

Regular envelopes are rectangular in shape and provide a long end for the machinery to grab for processing. As the envelope travels, machines locate the stamp and address, but such processing is made more difficult or impossible when the envelope is square or not a typical size. Such envelopes are better handled by people, which increases processing costs.Learn more about Postal Services