**Yes—some services advertise automatic booking at a target price, though availability and booking rules vary.** What you describe is essentially a *limit buy order* for a flight.

The closest examples I found are:

| Service | How closely it matches |
|---|---|
| [Set My Fare](https://setmyfare.co.uk/how-it-works.php) | Advertises the exact workflow: choose a flight, set your target price, authorize your card, and let it automatically book when the fare reaches your target. I could verify its indexed description, but couldn’t confirm its current operation or supported airlines. |
| [AirScalp](https://www.airscalp.com/) | Describes rules covering route, dates, cabin, stops, passengers and a hard price ceiling, with optional automatic booking. **It is accepting early-access requests**, so treat this as an announced product. |
| [Nomadiq](https://nomadiq.co.in/terms) | Offers automatic booking after monitoring fares. Its domestic-flight terms require advance funding at the current fare and say it will eventually book even if the desired drop never happens. That differs from your “buy only below XXX” condition. |

If an alert is sufficient, [milo](https://milo.flights/) supports a target price plus criteria such as date windows, trip length, stops and airlines. You complete the purchase yourself.

**The evidence points to booking and payment constraints, rather than a specific prohibition on the idea.** I didn’t find a rule specifically banning conditional flight purchases. There are several concrete complications:

- **A displayed fare can change before purchase.** Duffel’s airline-booking documentation explicitly says that retrieving an offer does not guarantee availability at booking, and that its price can change. An automatic buyer must recheck the final price and stop if it exceeds your limit. [Duffel documentation](https://duffel.com/docs/api/offers/get-offer-by-id)
- **The service needs a way to issue tickets.** For example, Amadeus requires users of its self-service booking API to contract with a ticketing consolidator before obtaining production access. Its requirements also include meeting local legal obligations. [Amadeus FAQ](https://admin.developers.amadeus.com/self-service/apis-docs/guides/developer-guides/faq/)
- **Charging your card later requires authorization and payment handling.** Payment systems support merchant-initiated transactions under an agreement with the customer, but authentication requirements can complicate unattended purchases. [Stripe’s explanation](https://support.stripe.com/questions/merchant-initiated-transactions-%28mits%29-when-to-categorize-a-transaction-as-mit)
- **Your criteria need to define the whole purchase.** “Below $400” needs to specify whether that includes bags and fees, and which fare restrictions you accept. Booking APIs expose separate ancillary services and change/refund conditions, so the automation must evaluate those too. [Duffel offer documentation](https://duffel.com/docs/api/v2/offers)

My inference is that these obligations help explain why alerts are easier to offer: automatic buying commits the service to executing and supporting a ticket purchase. The concept is feasible; the exact “buy below my ceiling, otherwise do nothing” behavior needs careful checking in each service’s terms.