When you are logged in as an administrator and booking a new appointment, there is a collapsed "Add-Ons" section available. You can expand this section to easily include any relevant add-ons to the appointment. This will show any add-ons from the booking form that's selected.
Clients can select add-ons when booking. In Forms > edit the form > add an "Add-Ons" question. Add-ons can be set to increase the price charged to clients, add extra time, and you can also optionally allow clients to select a quantity to purchase multiple of the same add-on.