# File lib/active_merchant/billing/integrations/valitor/helper.rb, line 48
          def product(id, options={})
            raise ArgumentError, "Product id #{id} is not an integer between 1 and 500" unless id.to_i > 0 && id.to_i <= 500
            requires!(options, :amount, :description)
            options.assert_valid_keys([:description, :quantity, :amount, :discount])

            add_field("Vara_#{id}_Verd", format_amount(options[:amount]))
            add_field("Vara_#{id}_Fjoldi", options[:quantity] || "1")
            
            add_field("Vara_#{id}_Lysing", options[:description]) if options[:description]
            add_field("Vara_#{id}_Afslattur", options[:discount] || '0')
            
            @products ||= []
            @products << id.to_i
          end