Do they take CCs? That would solve the change problem, wouldn't it? Though CCs are only useful if the airline is geared up to do the transaction quickly. Chip+PIN in the UK is now so fast in many shops that CC transactions are much less of a hassle than they used to be, so I'm using my CC for ever-smaller sums.
But the best solution is surely to avoid flying with carriers that have buy on board? (I'll get my coat.)