After PHP 7 upgrade
Warning: A non-numeric value encountered in /home/mecha4ma/public_html/wp-content/plugins/woocommerce-shipping-flat-rate-boxes/includes/class-wc-shipping-flat-rate-boxes.php on line 344
Line 344:
// add fee
$cost += $this->get_fee( $this->fee, $package->value );
Change to:
// add fee$cost += (float)$this->get_fee( $this->fee, $package->value );