Whenever I get into an airport taxi I always mention "Oh you take credit card right?" due to the queue system at airports they either reply yes and, if they actually don't, I get out guilt free at the other end - or they reply no and I move to the next taxi down the line. A significant number of airport taxis do shady stuff to try and extract extra or under the table payment.

(Edit, just to clarify - I get out of the Taxi on the other end guilt free because the driver lied to me about payment options. I don't like skipping out on service payments - I think people should be paid fairly for the work they're doing (even if I could get away with not paying)... but if you're lying to me you're doing me a disservice)