Just checked a Wizz flight for £16.99. Then found there is a £7 'Booking Fee' then £7 for paying by Visa.
With any airline, you really need to go to the end of the booking process to find out exactly what you're paying! But as you say, it is a pain for the single traveller - I would assume these add-on fees wouldn't change if I were with my family, and from what you say it's the same with Easy.