Frequent flyers and consumer-rights advocates often complain about the convoluted nature of airline ticket prices. Discount tickets can quickly become expensive through the use of “drip pricing,” extra fees for once-free amenities such as checked bags, the good snacks or a seat next to your child. And a dizzying array of fare classes with “dynamic” prices that shift over time has made it much harder to get a deal by planning ahead, or even to know whether the price you’re offered today is better...