Hello Daniele,

Thanks for reaching out to us.

Please, find the answers below

- Is it possible to create a service that can only be booked by the administrator and not by site visitors?

Unfortunatelly, by default, no. Any membership plugin (like MemberPress, Ultimate Member, etc.) allows you to create pages that are inaccessible to some (for example non-logged-in users). 

You can also create custom logic, so a user can access this page once a month (for example), but we're not sure about that, since we don't work with Membership plugins. 

- Can prices be changed for a predefined period without using events?

Unfortunatelly, there is no option to set the custom pricing per date range, as you have for Events, but the workaround would be to create a new employee, set the working hours (for the predefined period) and assign to the same service where the other employee is assigned. You just need to change the pricing for this employee and the customers will have two options where they can choose the "standard" pricing and the "predefined date" pricing.

I hope i cleared a bit.