Can I hold a seat for my customer before they pay for the ticket?

Sometimes yes, but it depends entirely on the airline and the fare you have chosen. Where an airline allows it, the flight booking API gives you back a booking reference with a ticketing deadline, which means the seats are reserved in your customer's name while payment is still outstanding. You can keep that unissued booking in the software, send the itinerary to your customer, and issue the ticket later once the money reaches you.

The deadline is set by the airline, not by glideFares. Many full service carriers allow a short hold on normal published fares, promotional and heavily discounted fares often have to be ticketed immediately, and most low cost airlines do not offer a hold at all because their systems take payment at the moment of booking. A few airlines sell a paid hold as an optional extra. This means two customers on the same route can get very different answers depending on which flight and fare they pick.

It is also worth knowing that a hold protects the seat, not always the price. Taxes and fare conditions can be refreshed at the moment you come back to issue, and if the deadline passes without ticketing the airline will normally cancel the reservation quietly, without telling your customer. The software shows you the deadline against the booking so you can chase the balance in time, but it cannot stretch a limit the airline has already fixed.

Before you promise a hold to a customer, create the booking and check the ticketing deadline shown against it, and confirm with your supplier which airlines in your own setup allow holds at all, since that varies with your plan and the agreements behind your account.