Option for Staff's "Additional cost" to be added to time slots

[Valentina R]

Example of how it works: Product price is 100€. Staff has additional price of 10€. Product has an addon to extend the time for free. No matter if the addon is selected or not, the price with staff is 110€.

Example of wanted behaviour: Same settings but when the addon with extra time is selected, to multiply the staff’s additional price by the length of the total duration. If the base duration is 1h and total price is 110€, when selecting the 2h duration, the price would be 120€.

Requested here.