Aside from the many unanswered questions about USPS shipping, I found ONE real gem with all kinds of reasons why USPS shipping might not work:
1) First you register at usps.com
2) They'll email you a testing server, assuming that you are a developer. IGNORE everything about TESTING, but not the user name:
Thank you for registering for the U. S. Postal Service's Web Tools Application Program Interfaces (APIs). We are providing you with a User ID that serves multiple purposes, as explained below.Your Username is xxxxYour Password is xxxxYour Web Tools User ID, shown above, is required to test and integrate USPS Web Tools APIs. With this ID, you may begin sending calls to the test server. Depending on the API, the address to the test server is either http://testing.shippingapis.com/ShippingAPITest.dll or https://secure.shippingapis.com/ShippingAPITest.dll. Use this information in combination with your User ID and your XML string to send a request to the USPS servers. For more details, refer to the programming guides (located at http://www.usps.com/webtools) for the specific API you are integrating.A sample test request would look like: "http://testing.shippingapis.com/ShippingAPITest.dll?API=[API_Name]&XML=[XML_String_containing_User_ID]"...
3) Unless you do want to develop something, immediately let them know that you're using Magento.
4) They'll email the REAL server
5) Then you might just add the server to the shipping setup at ...index.php/admin/system_config/edit/section/carriers/ as I did.
I just tested it and it didn't work. So I went back to the Magento thread and looking at a screenshot there, I noticed that I hadn't entered my user ID. It's been so long, I forgot what it was and I looked it up in my notes.
It STILL didn't work.
FINALLY I realized as I went through their emails that my user ID is NOT what I used to set up my online web tools account at USPS, they had sent a separate user id and pw. I'd forgotten about that and didn't put it in my notes.
So with the RIGHT user id it finally works.