Recently a number of major airlines have started to roll out Basic Economy fares. They’re less expensive than your typical airfare, but they sometimes come with a catch: they don’t include carry-on bags. And if you wait until you’re at the gate, checking that bag can be twice as expensive.

That detail is spelled out when you buy a ticket, but if you overlook the fine print of the less-expensive fare and show up at the gate with that larger bag anyway, you can (and will) be forced to check it. Unlike gate-checked bags when the overhead is simply just full, gate-checking when you’re on a Basic Economy fare can get pretty expensive.

For now, American and United are the only two airlines that don’t include a carry-on in their Basic Economy option. You can only bring a personal item such as a purse or backpack on board with those fares. The Wall Street Journal reports that some customers are getting caught off-guard when they get to the gate and are asked to pay.

Gate-checked bags come not only with a $25 charge (or $35 if you’ve already checked another bag), but also a “gate-handling charge,” an additional $25 you need to pay for the hassle of having a gate-agent check the bag last minute while people are boarding. That additional $50-$60 can often kill any savings you got by buying the cheaper fare in the first place.

The solution? Make sure you know what fare you’ve purchased before you arrive at the airport. If you do buy a Basic Economy fare, either check your bag when you get to the airport, or be prepared to pay once you get to the gate.