Ship

http://code.google.com/p/php-parcel-tracker/source/browse/carriers/usps.class.php?r=ecd3a1e9dfed7a2555b39178bf6bfd197040933e


http://forum.opencart.com/viewtopic.php?f=119&t=35867


http://lamp-dev.com/usps-web-tools-guide/205


http://bytes.com/topic/php/answers/905483-usps-tracking-api