Filter hook in assign shipment for client.

add_filter( 'wpcargo_assign_email_recipients', 'custom_wpcargo_assign_email_recipients', 10, 4);
function custom_wpcargo_assign_email_recipients( $recipients, $post_id, $user_id, $designation ){
	$recipients .= ',emailtest@gmail.com';
	return $recipients;
}

Filter hooks in shipment change/update status for client.

add_filter( 'wpcargo_client_email_recipients', 'custom_wpcargo_client_email_recipients', 10, 3 );
function custom_wpcargo_client_email_recipients( $recipients, $post_id, $status ){
	$recipients .= ',emailtest@gmail.com';
	return $recipients;
}

Filter hooks in shipment change/update status for admin.

add_action( 'wpcargo_admin_email_recipients', 'custom_wpcargo_admin_email_recipients', 10, 3 );
function custom_wpcargo_admin_email_recipients( $recipients, $post_id, $status ){
	$recipients .= ',admintest@gmail.com';
	return $recipients;
}