/**
  * Map this object's API credentials to the PayPal NVP format for posting to the API.
  * 
  * Abstracted from @see get_payment_details_url for readability. 
  */
 protected function get_api_credentials_url()
 {
     return 'USER='******'&PWD=' . urlencode(PayPal_Digital_Goods_Configuration::password()) . '&SIGNATURE=' . urlencode(PayPal_Digital_Goods_Configuration::signature()) . '&VERSION=' . urlencode(PayPal_Digital_Goods_Configuration::version());
 }