Logic for adding items and validating them

checkout.rb

@@ -0,0 +1,28 @@
+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