you do not want to change these settings in the WA_eCart/WA_eCart_USPS_PHP.php file, edit these settings in the USPS Shipping server behavior that is applied to the confirm.php page by the checkout wizard.
next to the ship to zip, click the lightning bolt and select the eCartCheckoutForm_shipping_postcode session variable.
for the weight, click the lightning bolt next to weight and select the total weight column from the ecart section of the bindings panel.