Resort fees have gotten a big embrace by hotel operators in recent years. They’re estimated to have brought in up to $2.93 billion in 2018 alone. The average fee climbed 11% last year to $21 per night. And in high-end destinations like Miami or Hawaii, it can often run much higher.

These surcharges can become a blight for many guests. Hotels say they’re meant to help cover amenities such as pool, gym, or internet access. But the fees are not part of a room’s advertised base rate, which can lead to an unexpectedly high bill for travelers after arrival.

Related: Booking.com to charge commissions on resort fees