To facilitate the correct delivery of correspondence and possibly take advantage of reduced mailing rates, the 9-digit zip code should be used. There are several different places in which this information can be found:  

• Call the vendor. Everyone is supplied with a 9-digit code and most business correspondence includes the complete zip code.
• Call the post office. The information is available and someone there can assist with the last four digits of the zip code.
• Order and use a book which lists all zip codes plus the last four digits. One such vendor is National Customer Support Center (the short name is Natlcustomersupportcenter). The vendor number is 941308560 B. The cost of the book is $18.00 and the company accepts purchase orders as well as prepayments.
• Use the internet. The address to locate zip codes is: http://www.usps.gov/ncsc/lookups/lookup_zip+4.html