Service extras during the appointment

We sometimes upsell further products/services during the actual appointment itself. These extra services are only made available during the appointment as it depends on the assessment during the appointment, so I cannot show them before the booking. This means I cannot use the current extra services module. It would be great if you can have the option to "hide" the extra services from the front end, so then I could create it as an extra service, from the front end it can't be seen, and then if the client has the extra service in the appointment, we can edit the appointment and add the extra service, so then it shows up in their invoice too.