In order to use USPS Rates API you need to first register with USPS.

1. Go to https://registration.shippingapis.com/ and fill out the registration form.
    
2. You will then receive an automated email that looks like this.
   
   
    
3. Take the Username and enter it into the USPS Settings in your store Admin.
    
4. Now you need to send USPS support an email requesting that your profile is switched to the production server. Use the following:
   
   

   Support,
   
   I have received my automatic email registering me with Web Tools Application Program Interfaces (APIs). As explicitly stated in the email, "When you have completed your testing, email the USPS Internet Customer Care Center (ICCC). They will switch your profile to allow you access to the production server." My website does not require any testing. It's an eCommerce website running CFWebstore. This cart is pre-configured to interface with USPS.
   
   Please switch my account to the production server so our 3rd party application can work correctly.
   My username is: xxxxxx(put your username here)
   Website is: www.yourwebsite.com
   
   Thank you.

   
   
    
5. USPS will generally respond within 48 hours. Once you're moved to the production server rates should appear in your shopping cart.