200 Commercial Letters, Flats, and Parcels Design ... USPS USPS Quick Service Guide referring to USPS' guide. referring to USPS' guide. Folded Self Mailer Chart Look Up non-machinable characteristics referring to USPS' guide. 200 Commercial Letters, Flats, and Parcels Design ... referring to USPS' guide. http://pe.usps.com/text/dmm300/201.htm#wp1042477 Physical Standards for Machinable Letters and Cards Physical Standards U.S. Post Office: Physical Standards