Done with class_eval

using_class_eval.rb

class Cart
  def confirm
    "Your order is being processed"
  end
end

cart = Cart.new

cart.class.class_eval do
  def deny
    "Your order has been denied!"
  end
end

cart.deny
#=> "Your order has been denied!"

next_cart = Cart.new
next_cart.deny
#=> "Your order has been denied!"