Extend plugin Class through functions.php

Question

Move manual fix applied to plugin source code to theme functions.php or as Code Snippet

I've applied a manual fix to a plugin file.

I'd like to make this fix more accessible and scalable by hooking this through the theme's functions.php or adding it as a snippet using a plugin like Code Snippets.

The Manual Fix applied to plugin file (functions as expected):

/**
         * Add admin meta box with actions
         *
         * @access public
         * @return void
         */
        public function add_woo_pdf_metabox()
        {
            global $post;

            if (!$post) {
                return;
            }

            /* Manual Fix Applied */
            $post_type_order = get_post_type( $post );
            echo $post_type_order;

            if ($post_type_order != 'shop_order') {
                return;
            }
            /* End Manual Fix */

            $order = new WC_Order($post->ID);

            if (!$order) {
                return;
            }

            $invoice = $this->get_invoice($post->ID);

            if ((is_array($invoice) && !empty($invoice) && $this->opt['woo_pdf_enabled']) || (!is_array($invoice) && $this->opt['woo_pdf_proforma_enabled'] && ($order->status != 'completed' || $this->opt['woo_pdf_enabled'])) || ($this->opt['woo_pdf_enabled'])) {
                add_meta_box('woo_pdf_metabox', __('PDF Invoices', 'woo_pdf'), array($this, 'woo_pdf_metabox_content'), 'shop_order', 'side', 'default');
            }
        }

Adjusted fix saved as Code Snippet using keyword extends on plugin Class (does not function):

add_action('plugins_loaded', 'regulate_pdf_metabox_appearance'); // hook used to call function

function regulate_pdf_metabox_appearance() { // wrapped in function
  if (class_exists('WooPDF')) {

    class WooPDF_custom extends WooPDF {

      /**
      * Add admin meta box with actions
      *
      * @access public
      * @return void
      */
        function add_woo_pdf_metabox() {
          global $post;

          if (!$post) {
            return;
          }

          /* Manual Fix Applied */
          $post_type_order = get_post_type( $post );

          if ($post_type_order != 'shop_order') {
            return;
          }

          $order = new WC_Order($post->ID);

          if (!$order) {
            return;
          }

          $invoice = $this->get_invoice($post->ID);

          if ((is_array($invoice) && !empty($invoice) && $this->opt['woo_pdf_enabled']) || (!is_array($invoice) && $this->opt['woo_pdf_proforma_enabled'] && ($order->status != 'completed' || $this->opt['woo_pdf_enabled'])) || ($this->opt['woo_pdf_enabled'])) {
            add_meta_box('woo_pdf_metabox', __('PDF Invoices', 'woo_pdf'), array($this, 'woo_pdf_metabox_content'), 'shop_order', 'side', 'default');
          }
        }
    }

  }
}

What is required in the code above to have the same fix work using extends and inserting it into the theme's functions.php or through a Code Snippet?