Online shoppers waste time searching for and testing multiple coupons at checkout. A browser extension could automatically detect valid coupons and apply the best combination for each purchase.
Integrate with major retailers' APIs to fetch valid codes and apply them programmatically at checkout. Offer a premium tier that finds hidden loyalty discounts.
The smallest version could be a Chrome extension supporting 2-3 major retailers with basic stacking logic. Biggest risk is retailers blocking automated coupon applications.