How to add fixed admin fee to service products

Hi, I am trying to figure out a way of creating a small, fixed fee as an admin/service fee (e.g. £1.50), that applies to all our service product purchases (bookings). This would need to apply automatically and not be an optional add-on, and to be one per order. So, if a customer booked three services in one order, they would only pay one admin fee.

We need to work out a way to do this that would not be affected, altered by discounts that may be applied via codes to the services themselves. 

Any help appreciated! Thanks