All,
 
If you're using the new USPS V3 production shipping API's, you need to change the server you're calling. Remove the "stg-" out of the external URL so it reads like this:
 
http://production.shippingapis.com/ShippingAPI.dll
 
Then, your cart will work properly.
 
Hope this gets out to you before your customers start howling. USPS discontinued the staging servers (stg-) last night.
 
Ed