It should take a amount due, tender. It should give an error and quit if it contains anything besides a number, except 1 ".". It should only have 2 decimals.

cashier_calculate.rb

class Calculator
 
  def amount_due(number)
    if invalid?(number) == true
      puts "Error message amount_due"
      exit
    else
      list = [number]
      
      while number != "done"
        @due = number
        
        if invalid?(number) == true
          puts "Error message amount_due"
          exit
        else
          puts "What amount is due?"
        end
       
        number = gets.chomp
        if number == "done"
          total_due = list.inject(0) { |sum, num| sum.to_f + num.to_f } 
        else
          list << number
        end
      end
        puts
        list.inject(0) { |sum, num| puts sum.to_f + num.to_f } 
        @total = total_due   
        puts
        puts "The amount due is $#{@total}"
    end
  end
   
  def amount_tender(number)
    if invalid?(number) == true
      puts "Error message amount Tendered"
      exit
    else
      @tendered = number
      total = @total.to_f - @tendered.to_f
      if @total.to_f < @tendered.to_f || @total.to_f == @tendered.to_f
        puts "===Thank You!==="
        puts "The total change due is $#{'%.2f' % total.abs}"
        today_time = Time.now.strftime("%m/%d/%y %R")
        puts today_time
        puts "================"
      else
        while total > 0 
          total = total - @still_owe.to_f
          if total.to_f < 0
            puts "===Thank You!==="
            puts "The total change due is $#{'%.2f' % total.abs}"
            today_time = Time.now.strftime("%m/%d/%y %R")
            puts today_time
            puts "================"
          elsif total.to_f > 0 
            puts "You owe total: $#{'%.2f' % total.abs}"
            @still_owe = gets.chomp
            if invalid?(@still_owe) == true
              puts "Error message amount Tendered"
              exit
            end
          else
            puts "You have no change due"
          end
        end
      end
    end
  end
   
  def invalid?(which_method)
    !which_method.match(/\A\d+(\.\d+)?\z/) 
  end
 
end
 
calculate = Calculator.new
puts "What amount is due?"
number = gets.chomp
total_due = calculate.amount_due(number)
puts "What amount is tendered?"
tender_amount = gets.chomp
total_tendered = calculate.amount_tender(tender_amount) 

Added a time stamp every time customers checkout.