In Store -> Shipping -> USPS Realtime Rates you could set up the realtime shipping rates via USPS web service.
Please go to https://secure.shippingapis.com/registration/ to sign up an account and get the User ID. Make sure your User ID set up for production server. Then you can fill out the form for USPS Realtime Rates. Be sure to check the check box "Send to Production Server".