(CNN) — When Barbara Nicolle of Vancouver Island, Canada, purchased a week-long, $10,000 trip to Mexico using the booking site Expedia, global pandemic was the last thing on her mind.

She bought her tickets last June. Now, as the coronavirus spreads, Nicolle is one of thousands of travelers scrambling to change their plans.

Nicolle had planned to travel with her husband, their three children and one additional minor, leaving for Mexico on Monday, March 16.

"There's talk that they may close the borders," she said on Sunday. "At that point we said that there's no way we could go." Today, Prime Minister Justin Trudeau announced the country's borders would close to many foreigners.

Now Nicolle, along with many customers of third-party sites like Expedia, Booking.com and Orbitz, has struggled to get assistance.

First, she tried to contact Expedia, where a surge of calls made customer service nearly impossible to reach. "I started to call them and sit on hold," she says. "Usually about an hour and a half to two hours, and [they] just cut you off. I got to five hours another time."

Both the airline and the hotel told Nicolle they couldn't help process her cancellation, sending her right back to Expedia.

Colorado resident Peter Milligan had a similar experience with Booking.com. Milligan had booked trips to both New Mexico and Brunswick, Maine, where he planned to attend Bowdoin College's graduation. When Bowdoin closed due to concerns about coronavirus, Milligan got in touch with the booking site.

"We contacted Booking.com and got 'well, it's not us, it's the property owner,'" says Milligan, who found information on the Booking.com website suggesting he call the hotels.

"The properties all come back and say 'well, it's Booking.com, it's their policy.' They're not going to give us our money back." (Booking.com has extended its Forced Conditions clause to require some hotels to refund money for coronavirus-related cancellations, but it does not automatically cover domestic travel in the US.)

Booking a hotel stay through a third-party booking site can lead to complications if you need to cancel. Marriott

"At Booking.com, you can't get through to anybody," says Milligan, who spent thousands of dollars on his cancelled trips, money that has not yet been refunded. "They say they'll get back to you within 48 hours [but] no one gets back to you."

The entire travel industry is struggling to keep up with a wave of cancellations related to coronavirus. Here's what you need to know about getting refunds from third-party sites, also known as Online Travel Agencies (OTAs):

How do third-party booking sites work?

"You always have the option to book directly from an airline, or you can purchase tickets from an Online Travel Agency," says Scott Keyes, a budget travel expert and the founder of Scott's Cheap Flights.

"Sometimes it's actually slightly cheaper on an Online Travel Agency. That's one reason folks might choose to book." OTAs are middlemen, Keyes explains, who earn money by taking a small commission on bookings.

Sometimes, these bookings are harder to cancel, partly because many OTAs save on costs by limiting their customer service. "When things go wrong like this it can be a little bit trickier if you have booked using OTAs rather than directly," says Keyes. "You have to go through the OTA, and it takes a little bit longer."

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The good news, he says, is that travelers may still have recourse to refunds.

"It varies somewhat from OTA to OTA. Most of them are now being slammed with cancellation requests and change requests," Keyes says. To help process the sudden onslaught of requests, Keyes notes that most agencies are asking travelers wait until just before their trip to call.

"Hold off to call until within 72 hours of your trip to make sure folks who have really near-term, upcoming travel plans are getting prioritized," he says.

How are the major OTAs dealing with coronavirus?

Every Online Travel Agency has their own policies on cancellations. Some, but not all, are making special exceptions due to coronavirus.

Cruises can be booked through third-party sites, but this way of booking through the "middleman" can be problematic when it comes to making travel changes. Shutterstock

Some travelers may be able to change or cancel their own reservations via the website, though the company is unable to change or cancel bookings with some international and budget airlines. Expedia is also waiving hotel change fees for non-resident travelers in a handful of countries. A special Cruise Support Page offers guidance for travelers, who are asked not to call unless their cruise is within 10 days.

The company Booking.com has extended its Force Majeure/Forced Cancellations policy to some areas affected by coronavirus-related travel restrictions. In these cases, Booking.com expects partners to refund prepayment and cancellation costs, and the OTA will waive its commissions. Domestic travel purchased through Booking.com is not covered by Force Majeure.

HotelTonight, an OTA owned by Airbnb, is offering a special coronavirus exemption from its cancellation policy . Reservations made on or before March 14, with check-in dates from March 14 through April 14, may be cancelled for a full refund. Exemptions will also be made for guests who have contacted coronavirus.

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On a page dedicated to coronavirus information, OTA Orbitz also notes that they're experiencing extremely high call volumes, and suggests travelers submit requests via a customer support form . The page does not mention any special policies or refunds for travelers affected by coronavirus.

Some travelers who booked through Priceline may be able to cancel online; otherwise, the company asks that travelers wait until 72 hours before the trip before calling in. Priceline has not announced any fee waivers beyond the individual policies of airlines and hotels.

In a coronavirus announcement on Hotels.com, the company directed travelers to manage bookings online, only contacting customer service for travel within seven days. (While Hotels.com does not charge cancellation fees, it note s that some hotels and vacation rentals do.)

What about the smaller OTAs?

Scott Keyes, of Scott's Cheap Flights, notes that in addition to the major OTAs, such as those listed above, there are many smaller OTAs with little name recognition.

"It tends to be a bit simpler and easier to get refunds, or to have a positive adjudication of your case, when you're working with a major OTA," says Keyes.

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"There are also hundreds of really small OTAs," he explains. With even less customer support, these small sites can be very difficult for travelers to navigate if something goes wrong.