Add a checkbox below a specific shipping method in WooCommerce

Question

I'm trying to add checkbox below specific shipping method, the checkbox shown but not only when I choose the specific method, this is the code I put in functions.php according to instructions that I read

add_action( 'woocommerce_after_shipping_rate', 'checkout_shipping_additional_field', 20, 2 );
function checkout_shipping_additional_field( $method, $index )
{
    if( $method->get_id() == 'flat_rate:5'){
        echo '<br>
        <input type="checkbox" name="shipping_custom_1" id="shipping_custom_1" value="1" class="shipping_method shipping_custom_1">
        <label for="shipping_custom_1">אם אני לא בבית אני מאשרת להשאיר את החבילה לי הדלת</label>';
    }

this is my page https://www.reuya.co.il/cart/