Once upon a time Ryanair used to charge for credit card bookings but not for debit card ones. Now they charge for all bookings (a slightly lower fee if you use a debit card), so you ALWAYS pay more than the advertised charge.

Considering credit card companies charge on a percentage basis, and considering Ryanair's fares are, on average, less than GBP 50.00 return, Ryanair are making quite a nice profit out of those surcharges which are way above cost. But I guess they've got to make money SOMEHOW - they can't make much out of their fares.