Activate shipping with USPS
If you've negotiated a discounted rate with USPS, then you can enter your User ID when you activate your USPS service in your Shopify admin so that the discounted rate appears to your customers at checkout.
Register for a USPS User ID
Register for a USPS Web Tools user ID by filling out the USPS Web Tools registration form.
When you have completed the signup process, USPS will mail you a user ID. Make a note of this, as you will need your USPS user ID to complete the activation in Shopify.
Your USPS Web Tools user ID is a combination of letters and numbers, for example
123ABCDE1234, that is automatically generated by USPS.
The user ID will give you access to package tracking, rate, and service delivery standards APIs with USPS when you register.