def shipping_cost_xml
      xml = Builder::XmlMarkup.new
      if @flat_rate_shipping
        xml.price(:currency => currency) {
          xml.text! @flat_rate_shipping[:price].to_s
        }
      else
        xml.price(:currency => @currency) {
          xml.text! shipping_cost.to_s
        }
      end
    end
def shipping_cost_xml
      xml = Builder::XmlMarkup.new
      if @flat_rate_shipping
        xml.price(:currency => currency) {
          xml.text! @flat_rate_shipping[:price].to_s
        }
      else
        xml.price(:currency => @currency) {
          xml.text! shipping_cost.to_s
        }
      end
    end
Generates the XML for the shipping cost, conditional on
 @flat_rate_shipping being set.