decorator.rb

class Delivery
  WEIGHT_COEFFICIENT = 1
  VOLUME_COEFFICIENT = 1

  def initialize(order)
    @order = order
  end

  def cost
    @order.weight * WEIGHT_COEFFICIENT + @order.volume * VOLUME_COEFFICIENT
  end
end

module Delivery::BubbleWrapped
  def cost
    super + 10
  end
end

module Delivery::UpToDoor
  def cost
    super + 15
  end
end

module Delivery::Dhl
  def cost
    super + SomeDhlApi.calculate(city)
  end
end


order = Order.new(10, 5, 1, 1, "Moscow")
delivery = Delivery.new(order)
delivery.cost #=> 15

delivery.extend(Delivery::Dhl)
delivery.cost #=> 115

delivery.extend(Delivery::BubbleWrapped)
delivery.cost #=> 125

delivery.extend(Delivery::UpToDoor)
delivery.cost #=> 140