Build a currency-aware trip budget tracker that updates with live exchange rates

Steps

1. Fetch live exchange rates from a currency API (e.g., Open Exchange Rates, Frankfurter, or ECB data feed) for the traveler's home currency against all currencies in their itinerary destinations; refresh rates at least once daily.
2. Store each booking and expense in both the transaction currency and the home currency equivalent at the time of booking; record the exchange rate applied so the conversion can be audited.
3. Implement a budget envelope per trip or per destination; as new bookings and expenses are added, compute the running total in home currency and compare against the envelope.
4. For future expenses (e.g., estimated daily spend), apply the latest exchange rate with a clearly labeled 'estimated at current rates' caveat rather than treating them as confirmed amounts.
5. Alert the traveler when projected total spend exceeds their budget, factoring in known future bookings plus an estimated daily spend per remaining travel day.
6. On post-trip reconciliation, compare actual charged amounts (from bank/card feeds) against estimated converted amounts to surface exchange-rate variance.