Once the speaker accepts the invitation it would be normal for the speaker (or the Bureau) to issue a contract and confirm details regarding the event. A deposit is usually required, with the balance due either just before the appearance or directly afterwards. A separate invoice for any expenses incurred would be issued directly after the event.
You (and/or) the speaker may well wish for either a conference call or a pre-event meet, to confirm all detail of the event, the logistics and the desired outcomes. It is expected that the event booker will either be or delegate a point of contact (POC) for the speaker to liaise with should they have need to do so, and that this person be available to meet them as they arrive at the venue.