They are using some Javascript, but you don't need it for the form. On http://www.usps.com/tools/calculatepostage/welcome.htm?from=home&page=0061calculatepostage, the pounds input is named "P" and the ounces input is named "O". The package types are a radio button group named "MT".

Perhaps you're talking about something else?

If you can use their web API, do that. They are pretty slow to respond with useable developer keys though, so you might have to stick with WWW::Mechanize for a bit.