Do my customers get their e-ticket straight away after they pay for a flight on my portal?

Yes, in most cases the ticket is issued automatically as soon as the payment goes through, and your customer receives the ticket by email without you having to do anything. The portal talks to the airline or the supplier, gets the ticket number back, and sends the confirmation out under your own agency branding.

That said, not every booking behaves the same way. Some airlines accept the booking first and only confirm the ticket a short while later, so your customer may briefly see a pending status before the ticket arrives. If your card payments go through a gateway that holds funds for checking, ticketing waits until that check clears. A few fare types, and certain suppliers, are set up so a person has to issue the ticket rather than the system doing it on its own. Low cost carriers are usually instant because the ticket is created at the moment of sale, while fares coming through other channels can vary. This really does depend on the airline, the supplier you are connected to and the way your account is configured, so it is not something anyone can promise will always be immediate.

On your side you will see the booking status clearly, so you know whether a ticket has been issued, is still pending, or has failed and needs attention. You can resend the ticket to a customer who has lost the email, and your staff can follow up on anything stuck rather than waiting for the customer to call you.

It is worth asking which of your fare sources issue automatically and which do not, then adding a short line at checkout telling customers what to expect, and running a live test booking on each supplier before you go live.