It would be good if there was a straight forward way to charge for premium appointment times via the portal.
The only way I can think of doing that at the moment would be to duplicate all appointment types in the portal settings but change the appointment charge to the premium charge - but this would cause a cluttered and unfriendly UI.
A better way would be to manage premium appointment charges via zones in the diary combined with a percentage markup or set fee. Potentially the fee could be set/applied to a zone type in the portal settings. Prices would be displayed as “from £[default_price]” and then updated on selecting appointment times in a set diary zone, with the difference being displayed alongside the available times in the diary.