Snippet to start label generation if status changed to processing from Admin. WOOCOMMERCE UPS SHIPPING PLUGIN WITH PRINT LABEL - https://www.pluginhive.com/product/woocommerce-ups-shipping-plugin-with-print-label/

functions.php

/**
 * Snippet to start label generation if status changed to processing from Admin.
 * Created at : 19 Sep 2018
 * Updated at : 19 Sep 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/varun-pluginhive/134e7cf7743da10b7fd3fe51121275eb
 */
if( ! function_exists('trigger_ups_label_generation') ) {

	function trigger_label_generation_externally( $order_id ) {
		$order 					= new WC_Order($order_id);
		
		$order_items = $order->get_items();
		if( empty($order_items) && class_exists('WC_Admin_Meta_Boxes') ) {
			WC_Admin_Meta_Boxes::add_error( __( 'UPS - No product Found. Please check the products in order.', 'ups-woocommerce-shipping' ) );
			return;
		}
		//  Automatically Generate Packages		
		$current_minute=(integer)date('i');
		$package_url=admin_url( '/post.php?wf_ups_generate_packages='.base64_encode('|'.$order_id).'&auto_generate='.md5($current_minute) );
		$ch = curl_init();
		curl_setopt($ch,CURLOPT_URL,$package_url);
		curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, FALSE);
		curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);
		$output=curl_exec($ch);
		if( ! $output && curl_errno($ch) ) {
			WC_Admin_Meta_Boxes::add_error( __( 'UPS - Curl error while automatic package generation. Error number - ', 'ups-woocommerce-shipping' ). curl_errno($ch) );
		}
		curl_close($ch);
	}

	function trigger_ups_label_generation( $order_id, $old_status, $new_status ) {
		if( is_admin() ) {
			if( $new_status == 'processing' ) {
				trigger_label_generation_externally($order_id);
			}
		}
	}
	
	add_action('woocommerce_order_status_changed', 'trigger_ups_label_generation', 10, 3 );
}