Zip Code 18040 Unacceptable Postal Names. 18040 is located in the state of Pennsylvania in the Philadelphia metro area. Zip code 18040 is primarily located in Northampton County. The official postal service name used for 18040 is EASTON, Pennsylvania.ip Code 18040 Unacceptable Postal Names. 18040 is located in the state of Pennsylvania in the Philadelphia metro area. Zip code 18040 is primarily located in Northampton County. The official postal service name used for 18040 is EASTON, Pennsylvania.