Can I hold a seat for my customer for a while before they pay, or do I have to issue the ticket straight away?

Yes, with many airlines you can hold a seat and issue the ticket a little later, but it is not something every airline offers. When the booking is made, the airline gives a ticketing time limit, and until that moment passes the seat sits reserved in your customer's name at the fare that was quoted. If the ticket is not issued before the limit runs out, the airline cancels the hold on its own and the seat goes back on sale.

The important thing to understand is that the time limit belongs to the airline, not to you and not to your portal. Some carriers give a comfortable window, some give only a very short one, and low cost airlines usually do not allow a hold at all and expect payment at the moment of booking. The fare also matters, because the cheapest promotional fares are often the first ones the airline excludes from holding. So you will see the option on some searches and not on others, and your portal can only pass on what the airline is willing to allow.

Whether you see the option at all can also depend on the supplier connection sitting behind your flight search, as some suppliers issue every booking straight away and never offer a hold. That is worth asking your supplier about rather than assuming it will be there.

Before you promise a customer that you will keep their seat, look for the ticketing time limit shown against that fare on the booking screen, and set yourself a reminder to either issue or cancel before it expires, because an expired hold usually cannot be brought back at the same fare.