Other Monks will no doubt be able to help with an LWP style solution to send form information to the USPS site, but I wanted to point out that there is another option than simply sending and scraping HTML. The USPS has APIs available that would no doubt be a less fragile solution than relying on the formatting of the public facing USPS web site. You want to visit:

1. The USPS Web Tools Registration Page to get a developer id
2. The Permission Request Form for USPS Web Tools to get authorization and information on using the APIs