Untitled

add_filter( 'pws_add_rate', function ( array $rate, array $package, PWS_Shipping_Method $method ): array {

    $is_on_backorder = false;

    foreach ( $package['contents'] as $item ) {

        /** @var WC_Product $product */
        $product = $item['data'];

        if ( $product->is_on_backorder() ) {
            $is_on_backorder = true;
        }
    }

    if ( $is_on_backorder ) {
        $rate['cost'] *= 2;
    }

    return $rate;
}, 1000, 3 );