Extending an existing Plugin class results in Class not Found PHP Fatal Error
There is no D8 commerce_usps module yet. I found a development version on github, I pressume you're using this one. 

The use statement uses the following structure use [namespace]\[classname]

So your use statement needs to be

use Drupal\commerce_usps\Plugin\Commerce\ShippingMethod\USPS

class ForExample extends USPS { ...