RELATED: Never Do This When Your Flight Is Canceled, Travel Expert Warns. A Sept. 21 TikTok video that claims buying flight tickets at the airport counter for some airlines may net cheaper prices went viral, earning more than 60,000 likes and one million views. But experts are now warning that for many major airlines, this advice is too good to be true. According to travel agency company CheapOair, buying a ticket for a United flight at the airport won’t yield cheaper prices and may end up costing you more, as airfares tend to rise over time. “United Airlines tickets are never cheaper at the airport,” the travel agency states. “Online flight booking can help you save big on airfare as flight booking sites offer various deals and discounts and promo codes on flight booking. You can make use of the market competition in your favor and can search for the cheapest tickets. On the other hand, it is very rare that one is able to get any exciting offers on flight booking at the airport.” RELATED: Never Do This If You’re on an Oversold Flight, Expert Warns. Experts say the same rules apply when booking a flight for another major airline: Delta. According to Delta’s website, there are several fees that could be imposed on top of the base fare for your ticket, but none of them are online-exclusive. The airline says they are “committed to offering you the lowest fare for Delta flights” when you book directly from them in any way, “including on delta.com or via the Fly Delta app.” In fact, if you find a lower price anywhere within 24 hours of purchasing your ticket, the airline says you can cancel it and receive a full refund. “Delta flights, when you buy at the airport, are never cheaper,” travel agency FlyCoAir confirms on its website. If you’re booking a ticket from Allegiant, Frontier, or Spirit, you may want to consider buying directly from the airport, however. According to Airfarewatchdog, an online site where experts monitor airfare prices, these three airlines do charge a fee for the convenience of booking online.ae0fcc31ae342fd3a1346ebb1f342fcb Allegiant Airlines has an “Electronic Carrier Usage Charge” which costs $18 each way when booking online but not at the airport, while Frontier imposes a $4 to $21 “Carrier Interface Charge” each way on nearly all of its fares booked online. Spirit Airlines’ online fee is called the “Passenger Usage Charge” and can cost people buying tickets online up to $22.99 each way. “These carrier-imposed fees go unnoticed by most because they are often lumped in with all of the government-imposed taxes and fees, which makes them seem like a mandatory charge,” experts explain. RELATED: For more travel advice delivered straight to your inbox, sign up for our daily newsletter. It’s essential to do your own research for each plane ticket you buy, as some of these budget airlines also offer online-exclusive discounts. According to Airfarewatchdog, Allegiant Airlines offers a “roundtrip discount” for tickets booked on its website, which might not get applied to tickets booked directly at the airport. This airline also offers discounted bag fees online, which you cannot get if you purchase your ticket at the airport. “If you need either a checked bag or a carry-on bag, this price difference for luggage will likely wipe out any savings you made by purchasing your ticket at the airport,” the experts say. Both Frontier and Spirit also offer online-exclusive bundles or discounts for some flights and charge higher bag fees at the airport. But these two airports might also assign your seats for free if you order your ticket at the airport. “For many, it is incredibly inconvenient to go to the airport just to purchase a ticket. Airports are usually hard to get to and generally far from city centers. Make sure to factor in your time getting to and from the airport and waiting in line to purchase a ticket. Add on to that the cost of more expensive bag fees, gas or transportation, plus any parking fees, and you may find the actual savings to not be worth the hassle,” the experts at Airefarewatchdog say. RELATED: Never Say These 2 Words to a Flight Attendant, Expert Warns.