diff --git a/boilerplate/python/services/checkout_service.py b/boilerplate/python/services/checkout_service.py
index 1669e31..16765dd 100644
--- a/boilerplate/python/services/checkout_service.py
+++ b/boilerplate/python/services/checkout_service.py
@@ -1,2 +1,147 @@
+# checkout_service.py
+from __future__ import annotations
+from dataclasses import dataclass
+from typing import List, Dict, Tuple
+import uuid
+
+# Reuse your catalogs
+from models import CATALOG, PLANS, ShippingOption  # CATALOG: phones, PLANS: plans
+
+# -----------------------------
+# Data contracts (internal)
+# -----------------------------
+@dataclass
+class CartItem:
+    sku: str               # "p-iphone-15" | "plan-basic" | "p-iphone-15|plan-basic"
+    kind: str              # "phone" | "plan" | "bundle"
+    qty: int = 1
+
+@dataclass
+class LineSummary:
+    label: str
+    amount: float
+
+@dataclass
+class Quote:
+    currency: str
+    items: List[LineSummary]
+    shipping: LineSummary
+    tax: LineSummary
+    monthly_total: float
+    grand_total: float
+
+# -----------------------------
+# Helpers
+# -----------------------------
+def _round2(x: float) -> float:
+    return float(f"{x:.2f}")
+
+def _price_for_item(sku: str, kind: str) -> Tuple[str, float, float]:
+    """
+    Returns (label, upfront_price, monthly_price) for ONE unit.
+    """
+    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 '{sku}', expected 'phoneSku|planSku'")
+        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}")
+
+# -----------------------------
+# Checkout Service
+# -----------------------------
 class CheckoutService:
-    pass
+    """
+    Stateless calculator for quotes & order confirmation.
+    Inject tax and shipping policy if you like; sensible defaults provided.
+    """
+
+    def __init__(
+        self,
+        shipping_options: List[ShippingOption],
+        currency: str = "EUR",
+        tax_rate: float = 0.08,     # 8% example
+        tax_on_shipping: bool = True,
+    ) -> None:
+        self.shipping_options = shipping_options
+        self.currency = currency
+        self.tax_rate = tax_rate
+        self.tax_on_shipping = tax_on_shipping
+
+    # public API --------------------------------------------------------------
+
+    def quote(self, items: List[CartItem], shipping_option_index: int) -> Quote:
+        if not items:
+            raise ValueError("Cart is empty")
+        if not (0 <= shipping_option_index < len(self.shipping_options)):
+            raise ValueError("Invalid shipping option")
+
+        # materialize line items
+        upfront = 0.0
+        monthly = 0.0
+        line_summaries: List[LineSummary] = []
+
+        for it in items:
+            if it.qty < 1:
+                raise ValueError("Quantity must be >= 1")
+            label, up, mo = _price_for_item(it.sku, it.kind)
+            up_total = up * it.qty
+            mo_total = mo * it.qty
+            upfront += up_total
+            monthly += mo_total
+            line_summaries.append(LineSummary(label=f"{label} ×{it.qty}", amount=_round2(up_total)))
+
+        # shipping
+        ship = self.shipping_options[shipping_option_index]
+        shipping_cost = ship.calculate_cost()
+
+        # tax (on upfront + optionally shipping)
+        taxable = upfront + (shipping_cost if self.tax_on_shipping else 0.0)
+        tax = taxable * self.tax_rate
+
+        grand = upfront + shipping_cost + tax
+
+        return Quote(
+            currency=self.currency,
+            items=line_summaries,
+            shipping=LineSummary(label=ship.name, amount=_round2(shipping_cost)),
+            tax=LineSummary(label=f"Tax ({int(self.tax_rate*100)}%)", amount=_round2(tax)),
+            monthly_total=_round2(monthly),
+            grand_total=_round2(grand),
+        )
+
+    def confirm(self, quote_request: Dict) -> Dict:
+        """
+        quote_request: {
+          "items": [{"sku": "...", "kind": "phone|plan|bundle", "qty": 1}, ...],
+          "shipping_option_index": 0
+        }
+        """
+        # compute a fresh quote to avoid tampering
+        items = [CartItem(**i) for i in quote_request["items"]]
+        q = self.quote(items, quote_request["shipping_option_index"])
+        order_id = uuid.uuid4().hex[:12]
+        # This is where you'd create a PaymentIntent (Stripe) or save to DB.
+        return {
+            "order_id": order_id,
+            "status": "created",
+            "currency": q.currency,
+            "grand_total": q.grand_total,
+            "monthly_total": q.monthly_total,
+        }