Hi,
There is indeed no option to do that. You have the Weight approximation and Dimension approximation settings that you can use to increase the weight/dimensions of the package and thus the shipping price calculated by USPS.
Otherwise, to really do what you want, you would have to add the line:
$usps_rate['Rate'] = $usps_rate['Rate'] * 1.2;
before the line:
if(isset($currency) && $currency != $rates[$type]->shipping_currency_id)
in the file plugins/hikashopshipping/usps/usps.php
Note however that you would loose the change after each update of HikaShop.