Hi,
This means that the USPS API provided a Media service without a "Rate" value.
It could be a bug in the USPS API following the changes they made.
To prevent that, what you can do is to add the code:
if(isset($rates[$mode]) && $rates[$mode]->shipping_price < 0.01) unset($rates[$mode]);
after the line:
$this->addRate($rates, $mode, $parcels, $rate, $currency, false);
(2 places) in the file plugins/hikashopshipping/usps/usps.php
It will prevent displaying any free shipping USPS shipping method on the checkout.