<P> Customers who send bulk mail can get a discount on postage if they have printed the barcode themselves and have presorted the mail . This requires more than just a simple font; mailing lists must be standardized with up - to - date Coding Accuracy Support System (CASS) - certified software that adds and verifies a full, correct ZIP + 4 Code and an additional two digits representing the exact delivery point . Furthermore, mail must be sorted in a specific manner to an 11 digit code with at least 150 mailpieces for each qualifying ZIP code and must be accompanied by documentation confirming this . These steps are usually done with PAVE - certified software that also prints the barcoded address labels and the barcoded sack or tray tags . </P> <P> This means that every single mailable point in the country has its own 12 - digit number (at least in theory). The delivery - point digits (the 10th and 11th digits) are calculated based on the primary or secondary number of the address . The USPS publishes the rules for calculating the delivery point in a document called the CASS Technical Guide . However, when confronted with two addresses like 18 and 18C, often CASS will assign the same 12 - digit number to two distinct mail delivery points . The last digit is always a check digit, which is obtained by summing all 5, 9 or 11 digits, taking the residue modulo 10 of this sum (i.e., the remainder after dividing by 10) and finally subtracting this from 10 . (Thus, the check digit for 10001 - 0001 00 would be 7, since 1 + 1 + 1 = 3, 3 ≡ 3 (mod 10) and 10--3 = 7 .) </P> <P> ZIP Codes designate delivery points within the United States (including territories), and overseas stations of its armed forces . There are also ZIP Codes for the independent countries the Federated States of Micronesia, the Republic of the Marshall Islands, and the Republic of Palau, each of which is integrated into the U.S. postal system under a Compact of Free Association . </P> <P> Mail to U.S. diplomatic missions overseas is addressed as if it were addressed to a street address in Washington, D.C. The four - digit diplomatic pouch number is used as a building number, while the city in which the embassy or consulate is located is combined with the word "Place" to form a street name . Each mission uses a ZIP + 4 Code consisting of 20521 and the diplomatic pouch number . For example, the mailing address of the U.S. Embassy in New Delhi, India would be: </P>

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