Snippet to add Company Logo in top of UPS label in ZPL format. WooCommerce UPS Shipping Plugin with Print Label by PluginHive - https://www.pluginhive.com/product/woocommerce-ups-shipping-plugin-with-print-label/

functions.php

/**
 * Snippet to add Company Logo in top of UPS label in zpl format.
 * Created at : 01 Aug 2018
 * Updated at : 01 Aug 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/xadapter/90f6b1f63b67fd4ef596b35adc940314
 */

if( ! function_exists('ph_generate_document_file') ) {
	function ph_generate_document_file($content, $file_name){
		
		$app_logo_file = "wp-content/uploads/aps-logo.zpl";			// Path of zpl company logo, relative to wordpress directory


		$uploads_dir_info		=	wp_upload_dir();
		$file_name_with_path	=	$uploads_dir_info['basedir'].$file_name;
		$app_logo_data = file_get_contents( ABSPATH.$app_logo_file);
		if( ! empty($app_logo_data) ) {
			file_put_contents( $file_name_with_path, $app_logo_data );
			$content = str_replace( "^XA^LRN^MNY^MFN,N^LH10,12^MCY^PW812^CI27", "LRN^MNY^MFN,N^LH10,420^MCY^PW812^CI27", $content );
		}
		file_put_contents( $file_name_with_path, $content, FILE_APPEND );
		header('Content-Type: application/octet-stream');
		header('Content-Disposition: attachment; filename='.basename($file_name));
		header('Expires: 0');
		header('Cache-Control: must-revalidate');
		header('Pragma: public');
		header('Content-Length: ' . filesize($file_name_with_path));
		readfile($file_name_with_path);
		unlink($file_name_with_path);
		return;
	}
}

add_action( 'woocommerce_after_register_post_type', function() {
	if( is_admin() && ! empty($_GET['wf_ups_print_label']) ) {
		$query_string		= explode('|', base64_decode($_GET['wf_ups_print_label']));
		$shipmentId 		= $query_string[0];
		$post_id 			= $query_string[1];
		$label_extn_code 	= $query_string[2];
		$index			 	= $query_string[3];
		$tracking_number    = $query_string[4];
		
		$label_meta_name='ups_label_details_array';
		if(isset($query_string[4])){
			$return			= $query_string[4];
			if($return=='return'){
				$label_meta_name='ups_return_label_details_array';
			}
		}
		$ups_label_details_array = get_post_meta( $post_id, $label_meta_name, true );

		$graphic_image = $ups_label_details_array[$shipmentId][$index]["GraphicImage"];

		if("ZPL" == $label_extn_code ) {
			$binary_label = base64_decode(chunk_split($graphic_image));

            // By default zpl code returned by UPS has ^POI command, which will invert the label because
            // of some reason. Removing it so that label will not be inverted.
			$zpl_label_inverted = str_replace( "^POI", "", $binary_label);
			
			$file_name = 'UPS-Shipping-Label-With-Company'.$post_id.'-'.$tracking_number.'.zpl';
			ph_generate_document_file($zpl_label_inverted, $file_name);
			exit;
		}
	}
});