diff --git a/boilerplate/python/services/cart_service.py b/boilerplate/python/services/cart_service.py
index 9555377..b5b3fe5 100644
--- a/boilerplate/python/services/cart_service.py
+++ b/boilerplate/python/services/cart_service.py
@@ -1,2 +1,158 @@
+# cart_service.py
+from __future__ import annotations
+from dataclasses import dataclass, asdict
+from typing import Dict, Tuple, List
+
+# Reuse your catalog dictionaries
+from models import CATALOG, PLANS  # {"sku": {"price": ...}} and {"sku": {"monthly": ...}}
+
+# -----------------------------
+# Data structures returned by the service
+# -----------------------------
+@dataclass
+class CartLine:
+    sku: str
+    kind: str          # "phone" | "plan" | "bundle"
+    qty: int
+    label: str
+    upfront_subtotal: float
+    monthly_subtotal: float
+
+@dataclass
+class CartTotals:
+    upfront_total: float
+    monthly_total: float
+    item_count: int
+
+@dataclass
+class CartView:
+    items: List[CartLine]
+    totals: CartTotals
+
+# -----------------------------
+# Pricing helpers (shared logic)
+# -----------------------------
+def _price_for_item(sku: str, kind: str) -> tuple[str, float, float]:
+    """
+    Returns (label, upfront_price, monthly_price) for ONE unit of the item.
+    """
+    if kind == "phone":
+        prod = CATALOG.get(sku)
+        if not prod:
+            raise KeyError(f"Unknown phone SKU: {sku}")
+        return prod["name"], float(prod["price"]), 0.0
+
+    if kind == "plan":
+        plan = PLANS.get(sku)
+        if not plan:
+            raise KeyError(f"Unknown plan SKU: {sku}")
+        return plan["name"], 0.0, float(plan["monthly"])
+
+    if kind == "bundle":
+        try:
+            phone_sku, plan_sku = sku.split("|", 1)
+        except ValueError:
+            raise KeyError(f"Invalid bundle SKU format (expected 'phone|plan'): {sku}")
+        phone_name, up, _ = _price_for_item(phone_sku, "phone")
+        plan_name, _, mo = _price_for_item(plan_sku, "plan")
+        return f"{phone_name} + {plan_name}", up, mo
+
+    raise KeyError(f"Unknown kind: {kind}")
+
+# -----------------------------
+# Storage interface (you can swap this for Redis/DB later)
+# -----------------------------
+class InMemoryCartStore:
+    """
+    carts[session_id] = dict[ (sku, kind) -> qty ]
+    """
+    def __init__(self) -> None:
+        self.carts: Dict[str, Dict[Tuple[str, str], int]] = {}
+
+    def get_cart(self, session_id: str) -> Dict[Tuple[str, str], int]:
+        return self.carts.setdefault(session_id, {})
+
+    def set_qty(self, session_id: str, sku: str, kind: str, qty: int) -> None:
+        cart = self.get_cart(session_id)
+        key = (sku, kind)
+        if qty <= 0:
+            cart.pop(key, None)
+        else:
+            cart[key] = qty
+
+    def clear(self, session_id: str) -> None:
+        self.carts[session_id] = {}
+
+# -----------------------------
+# Cart Service
+# -----------------------------
 class CartService:
-    pass
+    """
+    Business logic for cart management. Stateless; uses a store object for persistence.
+    """
+    def __init__(self, store: InMemoryCartStore | None = None) -> None:
+        self.store = store or InMemoryCartStore()
+
+    # ----- public API -----
+    def view(self, session_id: str) -> CartView:
+        cart_map = self.store.get_cart(session_id)
+        return self._materialize(cart_map)
+
+    def add(self, session_id: str, sku: str, kind: str, qty: int = 1) -> CartView:
+        if qty < 1:
+            raise ValueError("qty must be >= 1")
+        # validate by pricing once
+        _price_for_item(sku, kind)
+        cart = self.store.get_cart(session_id)
+        key = (sku, kind)
+        cart[key] = cart.get(key, 0) + qty
+        return self._materialize(cart)
+
+    def update_qty(self, session_id: str, sku: str, kind: str, qty: int) -> CartView:
+        if qty < 0:
+            raise ValueError("qty must be >= 0")
+        if qty > 0:
+            # validate only if setting positive qty
+            _price_for_item(sku, kind)
+        self.store.set_qty(session_id, sku, kind, qty)
+        return self.view(session_id)
+
+    def remove(self, session_id: str, sku: str, kind: str) -> CartView:
+        self.store.set_qty(session_id, sku, kind, 0)
+        return self.view(session_id)
+
+    def clear(self, session_id: str) -> CartView:
+        self.store.clear(session_id)
+        return self.view(session_id)
+
+    # ----- internal -----
+    def _materialize(self, cart_map: Dict[Tuple[str, str], int]) -> CartView:
+        items: List[CartLine] = []
+        upfront_total = 0.0
+        monthly_total = 0.0
+
+        for (sku, kind), qty in cart_map.items():
+            if qty <= 0:
+                continue
+            label, up, mo = _price_for_item(sku, kind)
+            up_sub = round(up * qty, 2)
+            mo_sub = round(mo * qty, 2)
+            upfront_total += up_sub
+            monthly_total += mo_sub
+            items.append(
+                CartLine(
+                    sku=sku,
+                    kind=kind,
+                    qty=qty,
+                    label=label,
+                    upfront_subtotal=up_sub,
+                    monthly_subtotal=mo_sub,
+                )
+            )
+
+        totals = CartTotals(
+            upfront_total=round(upfront_total, 2),
+            monthly_total=round(monthly_total, 2),
+            item_count=sum(qty for qty in cart_map.values() if qty > 0),
+        )
+        return CartView(items=items, totals=totals)