By using two separate classes Checkout, PricingRule and SpecialPrice, and especially using method count_price_for_items in PricingRule makes it easier to add new rules. In that case we can create new rules classes that will inherit from this PricingRule class, and override count_price_for_items method. It will come without code changes in Checkout class.
By creating PricingRule we can validate items that will be added to checkout.
I created three separate classes; each is representing different entity. I use private method pricing_rule_for in Checkout class, because it's only used in class method. It makes code more readable.