The 2014 extra postage fee for a square envelope is 20 cents for a 1oz envelope and 21 cents for a 2 oz envelope, in addition to the usual postage rates (additional fees may be charged for larger sizes). Because square envelopes cannot be processed by standard automated postal machines, they incur an additional fee and require special processing.
The United States Postal Service (USPS) classifies square envelopes as nonmachinable, meaning they cannot be processed by standard postal reading machines and must be handled manually. As such, square envelopes demand extra postage, called a nonmachinable premium. Typically, nonmachinable things have at least one of the following characteristics:
May jam postal equipment
May be challenging to process
Regular envelopes are rectangular and have a lengthy end for the processing gear to grab. As the envelope travels, machines locate the stamp and address, but square or non-standard-sized envelopes make this processing difficult or impossible. These envelopes are more easily handled by humans, which increases processing expenses.