Shopping Cart Pricer: Compute Order Totals with Discounts and Promo Codes

Question

## Round 1 - Coding

## Problem

Implement a shopping cart pricing engine. Products have base prices. Support:
- Quantity-based discounts (buy 3 or more, get 10% off that item)
- Promo codes that apply a percentage or fixed dollar discount to the cart total
- Free shipping above a subtotal threshold

```python
class ShoppingCart:
    def add_item(self, sku: str, price: float, qty: int) -> None:
        ...
    def apply_promo(self, code: str) -> bool:
        # returns True if code is valid
        ...
    def subtotal(self) -> float:
        ...
    def discount(self) -> float:
        ...
    def shipping(self) -> float:
        # 0 if subtotal >= free_shipping_threshold, else flat rate
        ...
    def total(self) -> float:
        ...
```

## Example

```
cart = ShoppingCart(free_shipping_threshold=50, shipping_rate=5.99)
cart.add_item("WIDGET", 10.0, 5)   # 5 units -> 10% qty discount -> 45.0
cart.add_item("GADGET", 20.0, 1)   # 1 unit, no qty discount   -> 20.0
cart.subtotal()   -> 65.0
cart.apply_promo("SAVE10")          # 10% off total
cart.discount()   -> 6.50
cart.shipping()   -> 0.0    # subtotal >= 50
cart.total()      -> 58.50
```

## Follow-ups

1. How do you apply discounts in the correct order — item discounts before or after promo codes?
2. How do you handle a promo code that is only valid on specific SKUs?
3. What happens if `apply_promo` is called multiple times — can multiple codes stack?
4. How would you serialize the cart state to restore an abandoned cart session?

Shopping Cart Pricer: Compute Order Totals with Discounts and Promo Codes

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