def subtotals
      x = {}
      charges.each do |r|
        x[r.product_code] ||= 0.to_f
        x[r.product_code]  += r.total_cost.to_f
      end
      return x
    end
def subtotals
      x = {}
      charges.each do |r|
        x[r.product_code] ||= 0.to_f
        x[r.product_code]  += r.total_cost.to_f
      end
      return x
    end
The total costs for the tag broken out by :product_code

 @return [Float]