Hi,
The USPS plugin is also pre installed with HikaShop which is why you can't find any information on how to download it separately.
In the Joomla plugins manager, search for "USPS" and you should find it.
If it's not there, then it has been uninstalled. You can take its files in the folder back/extensions/plg_hikashopshipping_usps of the HikaShop install package and zip them and then install that zip via the Joomla installer.
If it's there, then go in the menu System>Shipping methods, click on the "new" button and select the USPS shipping plugin and you'll be able to configure it.