Now imagine that you want to add a custom template from external source, not from that specific directory, then you can do that easily with this filter hook.

// Add New template from other directory
  function add_custom_template( $templates ){

      $templates[ 'custom-template' ] = array(
      	'label' 		=> __( 'Custom Template', 'text-domain' ),
          'thumb' 		=> 'URL of the thumbnail source',
          'stylesheet' 	=> 'Stylesheet of the template',
          'template' 		=> 'Directory of the template file',
  	return $templates;
  add_filter( 'wooin_invoice_templates', 'add_custom_template' );

Please see the dummy addon here