Can I hold a flight for my customer for a day or two before they pay?

Yes, on some airlines, but not on all of them, and your flight booking API will tell you which ones.

When you pick a fare, the airline behind it decides whether a seat can be held without payment. Many of the larger airlines that sell through the main airline reservation systems allow a booking to be created and left unticketed until a deadline they set. In that case the API gives you back the booking reference along with a ticketing time limit, so you can send your customer the itinerary while they make up their mind. A lot of low cost airlines work the other way round. Their fares are only confirmed once payment has gone through, so the only way to keep that price is to book and pay in one go.

The length of a hold is not something we decide. It comes from the airline, and it can be very short on busy routes, close to departure, or on heavily discounted fares. If the deadline passes without ticketing, the airline normally cancels the booking on its own, and the same fare may not be there when your customer comes back. So it is safer to treat a hold as a courtesy rather than a guarantee, and to make sure the deadline is clearly visible to whoever is handling the file.

It also helps to remember that a held booking is not a confirmed ticket, so your customer cannot travel on it or use it for a visa application. Before you build your sales process around holds, check which of your connected airlines allow them and what time limits they give, and ask our team to show you where the ticketing deadline appears in the booking details your API returns.