diff --git a/boilerplate/python/models/cart_item.py b/boilerplate/python/models/cart_item.py
index c9a62a4..3fdc481 100644
--- a/boilerplate/python/models/cart_item.py
+++ b/boilerplate/python/models/cart_item.py
@@ -1,2 +1,30 @@
+from typing import Literal
+
+Kind = Literal["phone", "plan", "bundle"]
+
+@dataclass
 class CartItem:
-    pass
+    sku: str
+    kind: Kind
+    qty: int = 1
+
+    def prices(self, catalog: dict, plans: dict) -> tuple[float, float]:
+        if self.qty < 1:
+            raise ValueError("qty must be >= 1")
+
+        if self.kind == "phone":
+            price = float(catalog[self.sku]["price"])
+            return price * self.qty, 0.0
+
+        if self.kind == "plan":
+            monthly = float(plans[self.sku]["monthly"])
+            return 0.0, monthly * self.qty
+
+        if self.kind == "bundle":
+            phone_sku, plan_sku = self.sku.split("|", 1)
+            up, _ = CartItem(phone_sku, "phone", self.qty).prices(catalog, plans)
+            _, mo = CartItem(plan_sku, "plan", self.qty).prices(catalog, plans)
+            return up, mo
+
+        raise ValueError("Unknown kind")
+