shipping_options.rb

class ShippingOption
  @options = []
  
  def self.inherited(child)
    @options << child
  end
  
  def self.for(weight, international)
    @options.select { |o| o.can_ship?(weight, international)}
  end
end

class MediaMail < ShippingOption
  def self.can_ship?(weight, international)
    !international
  end
end

class PriorityFlatRate < ShippingOption
  def self.can_ship?(weight, international)
    !international && weight < 4*16
  end
end

options = ShippingOption.for(3*16, false)
puts options.inspect # == [MediaMail, PriorityFlatRate]

options = ShippingOption.for(5*16, false)
puts options.inspect # == [MediaMail]