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You have to define Call url in shipment carries

"callback_url"=> base_url().'shipment/rates'

add these in controller or file which you add in call url

$filename = time();
    $input = file_get_contents('php://input');
    file_put_contents($filename.'-input', $input);

    // parse the request
    $rates = json_decode($input, true);

    // log the array format for easier interpreting
    file_put_contents($filename.'-debug', print_r($rates, true));

    // total up the cart quantities for simple rate calculations
    $quantity = 0;
    foreach($rates['rate']['items'] as $item) {
        $quantity =+ $item['quantity'];
    }

    // use number_format because shopify api expects the price to be "25.00" instead of just "25"

    // overnight shipping is 5 per item
    $overnight_cost = number_format(5, 2, '', '');

    // overnight shipping is 1 to 2 days after today
    $on_min_date = date('Y-m-d H:i:s O', strtotime('+1 day'));
    $on_max_date = date('Y-m-d H:i:s O', strtotime('+2 days'));


    // build the array of line items using the prior values
    $output = array('rates' => array(
        array(
            'service_name' => 'Shipment Local',
            'service_code' => 'SL',
            'total_price' => $overnight_cost,
            'currency' => 'PKR',
            'min_delivery_date' => $on_min_date,
            'max_delivery_date' => $on_max_date
        )
    ));

    // encode into a json response
    $json_output = json_encode($output);

    // log it so we can debug the response
    file_put_contents($filename.'-output', $json_output);

    // send it back to shopify
    print $json_output;

Change According to you needs