The “basic economy” fares recently unveiled by airlines including American and United to compete with ultra-low cost carriers also carry some of the unpleasant — and often unanticipated — fees associated with airlines such as Spirit and Allegiant.

Fortune reports that the cheapest fares offered by American Airlines and United don’t include a carry-on bag, with passengers travelling on these fares only being allowed to bring onboard small bags which fit under the seat in front of them.

Bags can be checked in advance, for a fee, of course. But travellers who show up with bags which only fit in the overhead baggage compartment are compelled to check them at the gate – at fees which can be twice as high as the cost of checking in advance.

For example, checking a first bag at the gate would cost $25, and a second bag would cost $35 — plus an additional $25 per bag “gate handling fee” imposed by the airlines.

The gate-handling fees “can often kill any savings you got by buying the cheaper fare in the first place,” Forbes noted.

Passengers can avoid the extra cost by checking their bags when tickets are purchased or upon arrival at the airport — just not at the gate.

The growth in basic economy fares being offered by full service carriers looks set to continue, with Aer Lingus having unveiled Saver fares for its transatlantic routes, and AA also considering the move.