Low-cost carrier Spirit Airlines is adding in yet another surcharge: This one, recently agreed to with the Department of Transportation, would allow the company to charge a "passenger usage fee" of $5 or $10 to anyone who has the nerve to buy a ticket anywhere other than one of the company's ticket counters.

In the past, I've actually spoken out in favor of a la carte pricing for airlines because it keeps the cost low for people who don't need services like beverages and checked bags.

But the problem with this fee is that it's a surcharge the vast majority of fliers will have to pay, meaning that it would be far more logical to offer a discount to travelers who book their flights at the company's kiosks. By tacking on the fee, Spirit Airlines is making price comparisons more difficult because according to the Wall Street Journal (subscription required), the fee will not show up in travel comparison shopping websites like Orbitz and Expedia.

If Spirit Airlines wants to be a low-cost leader, luring in customers with innovative advertising and a low-cost bare-bones business model, it shouldn't stoop to misleading tactics like hidden charges that are mandatory for all but a few customers.