Frontier Airlines charges for both checked and carry-on baggage. Additionally, they charge _more_ for luggage if you don't pay until you're at the airport, and even more if you wait til you're past security and at the gate.

Is it literally the exact same thing? No, but it definitely falls into the same category of sunk-cost/trapped buyers. Once you get to the gate, you're getting on the plane, even if you have to pay more for your luggage than you expected.

It's also very similar to why the soda vending machine at work charges $1 for a soda; the one in the amusement park charges $3, and the one _in the line_ for the popular ride costs $6. They all rely on facets of behavior that the consumer will just pay a little more once they've somehow invested in the process. The only difference is _what_ they invested and how easily recoverable it is.