Audit every payment flow in my app for client-trusted amounts. Find each place a charge is created and check where the amount, currency, and item list come from. The rule: the client sends only identifiers (product ID, plan ID, quantity), and the server looks up the price from its own data (database price list, the payment provider's price objects) and computes the total itself. Any amount, discount, or total arriving from the request body must be ignored or treated as a display hint that's re-verified. Apply the same rule to coupons and discounts: validate them server-side against active rules (expiry, usage limits, eligible products), not against a flag the client sends. Check cart math too: the server recomputes line items, tax, and shipping rather than summing client-provided line prices. Don't change the checkout UI or the displayed prices. Don't remove client-side totals used for display; just stop charging based on them. Deliver: each charge creation point, where its amount came from before, the server-side lookup now in place, and a tampering test: modify the price in the browser's network request (or send a crafted request with a lower amount) and confirm the charge uses the real price or gets rejected.