Adding a coupon/discount code system to your ASP.NET site requires a structured approach. Best practices include:

Database Setup – Store coupon codes, discount types, expiry dates, and usage limits.

Validation Logic – Ensure coupons meet conditions like cart total, user eligibility, and expiration.

Secure Implementation – Prevent abuse with redemption limits and logging.

Checkout Integration – Apply discounts dynamically and reflect them in pricing. For a quicker solution, third-party services like Voucherify, Talon.One, or Stripe/PayPal Discount APIs can be integrated.

You can also explore CouponDiscount.Codes for curated discount codes across various platforms.