Many community users report that the USPS shipping module doesn’t work well. It is due to the updated Price Calculator APIs of United States Postal Service (USPS). We implement a new USPS shipping module based on domestic rates v4 and international rates v2. It is now available at extension manager of TomatoCart free to download, at http://www.tomatocart.com/products/extension-manage.html?task=view&id=431.
It is necessary to get unique user ID from USPS so that you could access the online shipping services of USPS.
USPS official site teaches how to get the user ID at https://www.usps.com/business/web-tools-apis/development-guide-v3-1c.htm. Learn More