class Checkout
  class InvalidItem < StandardError; end

  AVAILABLE_ITEMS = {
    PI5: 60.00,
    ZERO2W: 15.00,
    PICO2: 4.00
  }

  def initialize
    @items = []
  end

  def add(item)
    raise InvalidItem unless valid_item(item)
    @items.push item
  end

  def items
    @items
  end

  private

  def valid_item(item)
    return AVAILABLE_ITEMS.keys.include?(item)
  end
end