def execute_purchase
      if credit_card_needed? or paypal_order?
        authorize_payment!
      end
      Product.connection.execute "savepoint quantity_update"
      product.decrement_quantity(self.quantity, self.product_style, self.product_size)
    rescue ActiveRecord::StatementInvalid => e
      Product.connection.execute "rollback to quantity_update"
      if e.message =~ /quantity_check/
        raise FulfillmentError, "not enough in stock"
      else
        raise
      end
    rescue StandardError => e
      errors.add_to_base e.message
      raise e
    end