Entities: Product, CartItem (product + quantity), Cart, Order, Payment. Sabse important design decision — CartItem mein price ka SNAPSHOT rakho, Product ka reference nahi. Agar seller kal price badal de to purane cart/order ka amount badalna BILKUL galat hoga.

Discounts ke liye Strategy + Chain use karo. Flat discount, percentage discount, BOGO, coupon — sab alag DiscountRule hain jo ek chain mein apply hote hain. Order of application matter karta hai (pehle item-level, phir cart-level) — ye clarify karna interviewer ko pasand aata hai.

class CartItem {
  private final String productId;
  private final Money unitPriceSnapshot;   // add karte waqt ka price, LOCK
  private int quantity;
}

interface DiscountRule {
  boolean applicable(Cart cart);
  Money discount(Cart cart);
}

class PricingEngine {
  Money finalAmount(Cart cart, List<DiscountRule> rules) {
    Money total = cart.subtotal();
    for (DiscountRule r : rules)
      if (r.applicable(cart)) total = total.minus(r.discount(cart));
    return total.plus(taxes(cart)).plus(shipping(cart));
  }
}