Add-ons are used to enhance a Service. They can include a cost but not a duration.
When booking online, a client can only select one Service with a duration. Along with that Service, they can also choose an Add-on that doesn't include a duration. A client cannot book two Services in one appointment if they both include a duration.
As the Business, you can book multiple Services that include a duration. When booking an appointment, select the first Service and its duration. Then, select to add another Service. You'll be able to select another Service with another duration. The appointment length will update to include both durations.
If you're using a Service as an Add-on and don't want it to be available for online booking on its own, you can hide it from your public Services page.
Additionally, you can add or extend an appointment's Recovery Time to accurately reflect the time needed.