Registering at USPS to use their shipping rates calculator is a two step process 

STEP ONE:

Go to https://www.usps.com/business/webtools.htm

Register:
You will see a link in the right frame page called: Register now to use USPS Web Tools .click that link and fill out the registration form.

Submit the form. You are now registered and USPS will send you an email with your User ID and Password.

STEP TWO:

After you receive your email with the User ID and Password, enter that information in the stores "Shipping Setup" Link under USPS.

To Complete your registration you will need to Call the USPS at 1-800 344-7779 Select the menu option for "Shipping API's" when a person answers you need to tell them that you are using a third party eStore that was the API's built into it and that you need to have your User name switched from their Test Server to their production Server. They will ask you for your User Name that they assigned you above. And they should switch you over to the production server.

Your registration in complete.

The Internet Customer Care Center (ICCC) is manned from 7:00AM to 11:00PM Eastern Time.
E-mail: icustomercare@usps.com
Telephone: 1-800-344-7779 (7:00AM to 11:00PM ET)