How to Add USPS Shipping Methods: 

1) Under "Products" Menu on the left hand side of the Builder

2) Select "Price Sets"

3) Click "Add" under "Shipping Methods"

4) Select "US Postal Service" from dropdown menu

5) Here on the "Edit Shipping Method" page, you can change the "Name" to reflect your chosen shipping method

*For Example: "USPS Standard Shipping", "USPS Expedited Shipping", etc.*

6)You can also change the "Service Level" to match the USPS Shipping method.