如何在没有钩子/过滤器的情况下覆盖/扩展WooCommerce类功能

I am using the WooCommerce Product Vendors plugin along with WooCommerce on a site. Within the plugin, there is a class in the file class-wc-product-vendors-commission.php with the function calc_order_product_commission().

I need to modify what this function does, however, there are no hook/filters for this function. It is not a template file I can override in my theme. I believe I need to use good ol' fashioned PHP to extend the class function. Here is the code pertinent to the class and function:

class WC_Product_Vendors_Commission {
   public $table_name;
   protected $masspay;

   /**
    * Calculates the commission for a product based on an order
    *
    * @access public
    * @since 2.0.0
    * @version 2.0.0
    * @param int $product_id
    * @param int $vendor_id
    * @param int $quantity
    * @return float $commission
    */
    public function calc_order_product_commission( 
       $product_id, $vendor_id, $product_amount, $quantity = 1 ) {


        // HERE IS WHERE I NEED TO CHANGE WHAT THIS FUNCTION DOES
        // AND MAKE COMMISSION A CUSTOM VALUE 


    return $commission;
   }
}

How can I either extend this class in my functions.php file or its own plugin to modify that function to do what I want? I do not want to edit the core plugin file.

THANKS!