Can I hold a flight seat for my customer before they have paid?

Yes, in many cases you can, but it depends on the airline and on the fare your customer has chosen. Some airlines allow a booking to sit unpaid until a deadline they set, and the flight booking API passes that deadline back to you along with the reservation. Until it runs out, the seats and the fare are held for your customer. If payment does not reach the airline in time, the reservation cancels itself and the seats go back on sale.

This is not offered on every fare. Low cost carriers and many discounted fares expect payment at the moment of booking, and for those the API will simply not return a deadline at all. Even where a hold is allowed, the airline decides how long it lasts, and it can be very short on a busy route or close to the departure date. The API only reports what the airline says, it cannot stretch the hold or argue for more time on your behalf.

Where you can use it, a hold is genuinely useful. It suits a family who need to check dates with each other before committing, or a company that has to approve the travel internally before it releases the money. You keep the reservation live in your system, chase the payment, and issue the ticket once the funds are in, without having to search and quote all over again.

Before you rely on this, ask your team to always read the deadline the API returns and to set a reminder against it, because once that time passes the fare is gone and the whole trip has to be priced again.