Getting Zip+4 for a list of addresses

I have a list of about 300 addresses with 5-digit zip codes and wonder
if there is a way to get the 5+4 zips without entering address manually
at www.usps.gov?

Thanks in advance!

Claude

http://pe.usps.gov/text/dmm200/discount2.htm

According to this page, "flats" have a minimum thickness of 1/4-inch. It
also says you can send newsletters as flats. I've never seen a
newsletter 1/4-inch thick. Is there an exception to the minimum
thickness for newsletters?

Claude