If you’re planning a vacation soon, money expert Clark Howard has a warning about third-party travel booking websites that you may run across while searching for cheap flights or hotel rooms.

Read more: The best and worst hotel chains in America

Booking a vacation? Beware of third-party travel booking websites

In 2015, the Better Business Bureau of Akron, Ohio, issued a special alert about JustFly.com, a site that specializes in low-cost airline tickets. The warning was issued again last year.

JustFly.com is part of Momentum Travel Group, which also owns another online travel agency called FlightHub.

Here are some of the complaints about JustFly.com, according to the latest BBB warning:

• Differences in the quoted price as opposed to what is actually charged on the consumer’s credit card after booking.
• Mistakes being made by booking agents, such as the name and spelling of the passenger’s name. Adjustments to the booking result in significant fees to the consumer from both the airline and JustFly.com.
• Differences in the time and date of travel from the original booking.
• Lack of email booking confirmations as promised.
• Reports of being on hold for excessive periods of time when calling into the customer service department. Some have reported being cut off after long waits.
• Being charged more for a cancellation fee than was originally quoted.

BBB said it identified a similar pattern of complaints against FlightHub, including customers who claim the price of their airfare went up after receiving a reservation confirmation.

As of May 2017, BBB lists FlightHub as unrated, while JustFly has a B+ rating.

Below is a screenshot of JustFly.com’s profile page on BBB’s website. Below the overall B+ rating, you can see that 98% of the customer reviews are negative.

Many of the stories from customers echo what was said in the BBB alerts from 2015 and 2016.

Important note: A BBB spokesman told Clark.com that customer reviews don’t contribute to the overall letter grade, but customer complaints submitted to the BBB do impact the grade.

That’s why you should never rely on a letter grade or star rating alone. Always read the reviews!

Listen to Clark discuss this topic on The Clark Howard Show Podcast

As Clark noted on the radio show, Kayak will link to some of these third-party booking sites that you may not be familiar with ‘ so you really have to do your homework.

Here’s what Clark recommends:

• Book directly with the airline or hotel
• Book through a travel agent
• Book using a travel website you know and trust

“With any hotel or flight booking, please be very careful who you go to because you never know what kind of trouble that will lead to,” Clark said.

Read more: Clark’s secret to finding the cheapest flights possible