The USPS User ID is not the USPS account User ID (Customer Account), it is actually an API Key to use the USPS API.
REQUEST A USPS USER ID:
USPS User ID - You can obtain your own user ID by registering for a Web Tools Application Program Interfaces (APIs).
The USPS account id is the id used for the Web tools account.
Some customers like to use there own. The default is Extension Works API account id.
If you are using your own USPS Customer ID please check the following:
USPS ID format: 123EXTTE2191 (12 Characters long)
API Settings -
In the admin area of our USPS extension if you do not add your own USPS user ID it will default to an account ID that was registered by Extension Works.
This is optional.
If you use your own USPS ID, you need to call or email USPS Tech support and have them move your account to ‘production’.