module InvoiceHelper
  def taxable? source
    if source == 'online' or source == 'AZ'
      true
    else
      false
    end
  end
end

class Sale
  include InvoiceHelper

  def initialize(total, source)
    @source = source
    @total = total
  end

  def place_order
    return (@total + (@total * 0.06)) if taxable? @source
    @total
  end
end