(KRON) – Frontier Airlines is facing some criticism over a policy that pays gate agents a bonus for catching oversize bags, and it’s getting attention on social media.

A source familiar with the situation tells Nexstar’s KRON that if the policy is being used to charge customers without cause, the airline could be breaking the law.

Flying a budget airline can have its advantages if you need the most affordable ticket or want to get somewhere last-minute for a low price. However, when the “hidden” fees start piling on, you may find yourself with a much bigger bill than you anticipated.

Customers flying on Frontier Airlines have brought their complaints about one such fee to TikTok, and the videos are going viral. One user shows how she and another customer’s personal bags both fit easily into the personal carry-on baggage checker at the departure gate. However, she and the other passenger were required to pay a $99 personal item fee when the gate agents said the bags were too big.

The user explains in later videos that she and the other traveler were denied boarding after filming their bags fitting into the baggage measurer. She said both had already paid the personal item fee when they were then denied entry onto the plane. Frontier’s baggage fees fluctuate depending on when you pay, with a fee booked online over 24 hours before the flight nearly half the cost of a fee paid at the departure gate.

When KRON reached out to Frontier Airlines for comment on these complaints, the media team said that the “carry-on bag size requirement” the airline had established was not “consistently applied” to all customers. The airline says this was unfair to the “majority” of customers who do pay for carry-on bags in advance.

Some commenters on the viral videos pointed out that Frontier Airlines has rolled out a new policy that reportedly pays gate agents a $10 bonus for each bag they can charge for at the gate. When asked about the bonus, a Frontier representative said it is only meant to “incentivize” their team members “to ensure compliance with the bag size requirements to ensure all customers are treated fairly, including the majority who comply with the rules.”

DOT’s Office of Aviation Consumer Protection is the main investigator of baggage complaints in regard to airlines. This agency provides a detailed dashboard for flyers to understand their rights when traveling with airlines.

An incentive program where agents can earn bonuses for flagging oversize bags doesn’t violate the law. However, if that incentive program ultimately causes agents to improperly classify a bag as oversized in order to charge an additional fee without cause, that would be a violation of the law, a source told KRON.

The DOT also lays out what consumers can expect from airlines that “involuntarily bump” them from a flight. The DOT website states that the first step is to file a complaint with the airline. If that does not resolve the issue, the customer can then file a complaint with DOT.

Frontier Airlines provided the following statement to KRON on the viral videos:

“Customers are clearly informed about our bag size rules during the booking process and are reminded prior to check-in and again during the check-in process. Signage at our ticket counters and gates also provide reminders. Bags can be purchased at any point prior to boarding at reduced cost.

As customers are boarding, those with personal items that may be oversized are asked to place their bags in the sizer. Bags must fit completely in the sizer without sticking out. It is also important to note that each customer is allowed only one personal item. Customers who wish to bring a second bag onboard, even if it is of a personal item size, need to purchase a carry-on bag.

We have on a number of occasions investigated specific claims by customers who say they were unfairly charged for a bag at the gate and typically find there is more to the story. All complaints are taken seriously and consumers are encouraged to contact our Customer Care team with any concerns.”