1) To set up shipping with USPS, you will first need to register an account with the U. S. Postal Service's Web Tools Application Program Interfaces (APIs). You can do that here: http://www.usps.com/webtools/. You will be sent a username and password that are each 12 characters long.
2) You will then need to send USPS an e-mail at firstname.lastname@example.org or call 1-800-344-7779 and ask them to switch your account from testing mode to the production server. In this e-mail you need to indicate that you are using MiiDuu.com as your third party software. If you forget to mention this, they will e-mail you again asking you to run tests or provide this information, and it will cost you more time.
3) Once you have received a confirmation that you have been switched to the production mode, sign into your MiiDuu account and navigate to the Shipping page (under the Setting tab). Click "Edit" link in the "Action" column of the USPS row and enter the username and password you were given when you registered with Web Tools and configure the settings to your needs. Don't forget to save your changes!
4) Go to your storefront and conduct a mock purchase to make sure everything is in working order. If it is, it should look something like this: