Go to Booking →settings → extras

Click on New Extra

Give the extra a name

Set the price

Chose the display order

Set if the extra is quantity based

Enter how many minutes and/or hours will take to complete the extra

Link the services that will have this extra available

Upload an icon, the recommended size and format is 55pxx55px PNG with transparent background

Click create

Go to booking → extras

Choose an extra and click the edit icon

Edit what you must and click update

Go to Booking →settings → extras

Select an Extra and click on the delete icon

Go to booking → extras

Choose an extra and click the edit icon

In the section "Choose Services" Add or remove the service(s) you want this particular extra to have.

This is useful when you want only certain services to have a specific extra.