Deferred-interest and 0% financing traps

Pain point

0% medical/retail financing retroactively charges all deferred interest if you miss the payoff window. People actively planning to arbitrage these (keep cash invested) don't know the fine print.

Signal

Moderate but recurring across contexts — the patient-financing post (though low score, ~2) drew an immediate warning, and the same "read the terms / set a reminder" theme appears in CD auto-renew (#5) and old-debt (#6). Low post volume but high consequence per user.

Opportunity

A "Promo APR Tracker" app: enter card/financing offer, promo end date, balance; it computes the required monthly payment to clear it in time, shows the exact dollar amount of deferred interest at risk, and escalates reminders near the deadline. Pairs naturally with #2's maturity alerts — same product. Feasibility: very quick to build; low moat, so bundle it.

Evidence

• Patient financing thread
• CD rates thread