This contribution has EXPLICIT instructions on adding USPS and how to do it and how to create an account.
http://www.oscommerce.com/community/contributions,487/page,6
You have to save and then read the text instructions with this download.
Yes you DO have to have an account with the USPS but it does't cost a thing, you just have to register.
To those who contributed this module - Thank You for going into detail with the instruction.
Only problem I had was selecting all options, didn't work. When I selected just the few that I needed it works like a charm!
-Kim