In order to add a fee to a booking, go to the Finance tab, click on More and select Add an Upsell. Then, a drawer opens in which you can select your desired fee from the drop-down menu.
Then, a drawer opens in which you can edit the selected upsell and finally click on Add.
The added fee will be shown on the Fees & Upsells card under the Details tab of the booking.
Note that you can also edit or reject this added fee here or on the Price Breakdown of the Finance tab.