Did you ever wonder why both Ryanair and Aer Lingus charge no fees for using certain payment cards?
By giving consumers a free option, the handling or administration fee becomes optional.
This is crucial as if there was no way out of paying the fee, this charge would have to be included up front in the breakdown of all charges.
Since November 2008, EU law obliges airlines to show an 'all-inclusive' final price at the beginning of the booking process and a breakdown of that price. But it only has to include charges that are 'unavoidable'.
This leaves plenty of scope for airlines to add a host of 'avoidable' extras later, ensuring that the initial total price doesn't look so high.