Form Post Trouble Using urllib

[Steve Allgood]

I'm having trouble posting a form at the USPS web site.  I've been
successful using urllib at other sites, but I'm missing why this won't
work:

# begin code
# get zip+4

import urllib

def zip4query():
    url = "http://www.usps.com/zip4"
    data = {
        'Selection': '1',
        'urbanization': '',
        'firm': '',
        'address': '',
        'address1': '1600 pennsylvania',
        'address2': '',
        'city': 'washington',
        'state': 'DC',
        'zipcode': '',
        }
    urldata = urllib.urlencode(data)
    results = urllib.urlopen(url, urldata).read()
    print results

zip4query()

# end code

I just get the query form back as my results. I didn't have this
problem before.  What am I doing wrong?

Thanks,
Steve