Activate shipping with USPS
If your store is eligible for Shopify Shipping, then real-time rates for USPS are enabled by default in your Shopify admin. If your store isn't eligible for Shopify Shipping, then you'll need to sign up with USPS before you can activate this carrier service on your Shopify store.
If your Shopify account is frozen at any time, your carriers might become deactivated. Check your shipping settings as soon as your account is restored.
Register for a USPS User ID
If your store is eligible for Shopify Shipping, then real-time rates for USPS are enabled by default in your Shopify admin. You do not need to register for a separate USPS User ID, and can get started by buying a shipping label for your next order.
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, e.g.
123ABCDE1234, that is automatically generated by USPS.
The user ID will automatically and immediately grant you access to package tracking, rate, and service delivery standards APIs with USPS when you register.
If you have any problems with your USPS setup, you must contact USPS directly. Shopify cannot obtain your USPS credentials for you, or provide support for your USPS account.
Activate carrier-calculated shipping for USPS
To activate carrier-calculated shipping for USPS:
From your Shopify admin, click Settings, and then click Shipping.
In the Carrier-calculated shipping section, click Manage carriers.
On the Real-time carriers page, find the USPS section, and then click Activate.
If your store is eligible for Shopify Shipping, click Save on the Enable USPS rates at checkout? dialog:
Otherwise, enter your USPS user ID into the form and click Save: