Before setting up your USPS shipping method, you will need the following:
A USPS User ID ; Note this is NOT the same as the user id you enter at usps.com.
You will need to register for a USPS User ID .
- In the Administration panel, go to Administration → Shipping & taxes → Shipping methods , or alternatively Setup your store -> Store management -> Setup shipping methods
then, Click "+" button to add shipping method
2. Give your shipping method a Name, and select the USPS shipping method in the Rate Calculation dropdown field 
3. Select from dropdown for example, USPS Priority Mail as Shipping service.
4. Enter a Delivery time . Note, this text (eg. 3-5 business days) will be overridden by the real-time shipping service.
5. Make sure that the Status of this method is set to Active.
6. Enter a Weight range. For example, the shipping service will calculate up to 4lbs. If a package is 6lbs, the shipping service will recalculate the overages and show this to the customer on checkout.
7. Click the Create button to save the changes. When you hit Create, more tabs will appear .
8. Switch to the Configure tab.
9. Enter your USPS credentials in the User ID field, enter your International USPS fields (if applicable), enter your Domestic USPS fields (if applicable), Then hit Save
Please note only some specific requests for retrieving shipping costs work properly when the Test mode option is enabled.
Note: If you have trouble with your USPS account, please visit the USPS Support Center and submit a USPS support desk ticket. Occasionally, their calculation service will be temporarily down for routine maintenance.
10. Switch to Test rate calculation tab . Here you can test your shipping calculations based on the Weight, Recipient, and Sender address. Then hit Recalculate rates, and SAVE.
