If the problem is the username that you have then you will need to contact USPS to get the correct value. The link that you provided looks ok to me.
The USPS file should be present in the WA eCart folder, I"m including a copy of it for you attached to this post. If you have a copy of this file anywhere in your site you should make sure to back it up before you add this file into the WA_eCart folder.
You can update the server behavior on the confirm page, in the server behaviors look for the shipping quotes (USPS) server behavior. When you edit it you should be able to adjust the username and other values that are used in it.