There is something wrong with the idea that airlines can charge 'booking' and 'payment' fees to a person using a debit card to pay government taxes when these taxes have to be paid as part of an on-line payment for a flight. The collection of these taxes is an obligation placed by Government of THE AIRLINES. Customers of airlines have to pay to the airline the tax content of their flight cost. The Airline then passes the tax content to the Government. There is no justification for the Airlines to charge customers a fee for collecting this tax, either in cash, by cheque, by debit card or by credit card.
Why is this idea important?
If what I say above is not true, then anyone at anytime, who is engaged in commerce, could levy charges in order to cover the cost of collecting Government taxes. I am almost sure that such a practice is forbidden by law, although I am not sure where in the law such a practice is forbidden.
The really serious point is that the Airline customer is NOT the person obliged to pay the tax. It is the airline which is obliged to pay the tax. To charge a fee to collect the tax is unlawful.