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Spirit Airlines Inc. (NASDAQ: SAVE) has a habit of both saving flyers money and irritating them with nickel and dime charges for goods and services, which can erode that savings substantially. Done correctly, a passenger can fly from New York’s La Guardia Airport to Detroit Metropolitan and back for about $126.

The first key is to pick the right day and travel one way. On December 6, which is a Tuesday, the price is about $63. The Sunday before, December 4, it is $25 more. The next Sunday, the 11th, the price is $70 higher.

If people don’t want to be upset by Spirit’s long list of extra charges, there are plenty of warnings from Spirit. The most important one is the core description of its business plan:

Our fares are fully unbundled. No “free” bag. No “free” drink. Other airlines bake those options right into their ticket price. We don’t. A ticket with us gets you and a personal item from A to B.

And:

We add more seats to our planes because when we fly with more people, the cost per person goes down. Our seats are simple: they don’t recline, so you don’t wind up with someone’s head in your lap. And we don’t have Wi-Fi or video, so be sure to download a movie to your phone or tablet. A little cozier seating and fewer expensive extras mean lower fares for you.

After wading through hotel and rental car options, Spirit gives a dizzying set of options. Add two bags, $70. For three bags, $155. In each case, that is the one-way charge. A wider seat is $75 each way. More charges for cups, napkins, boarding order and airport check-in.

It suddenly becomes less expensive to drive.