Given the endless rollercoaster of airfare, hotel and rental car pricing, most travellers stop looking for a deal after they’ve made their purchase.

But a new breed of US-based travel websites reduces the risk of paying too much by sending an alert when the price drops on something you’ve booked. You can then either rebook your flight, room or auto at the lower rate, or they will do it on your behalf, passing along the savings like a refund.

These services may seem "too good to be true" for some sceptical travellers, and certainly, none of these new tools replace the need for comparison-shopping across multiple sites to find the best deal. Yet I applaud these price-monitoring sites because they vastly increase the chances of a time-pressed consumer finding a good bargain. Here's a roundup of the newest players that pledge to notify you when you're owed money, though they only work for US reservations.

Hotel rooms
On 21 March, booking engine Tingo debuted with the goal of helping travellers lock in the lowest hotel rates. Prepay for your room through the site, and it will start monitoring rate changes for your reservation. If the hotel goes on sale on Tingo, the site will automatically rebook you at the lower rate and credit the difference to your credit card. For now, neither Tingo nor the hotel charges for the service.

Owned by parent company TripAdvisor, Tingo lists competitive prices for thousands of hotels, though not it’s nearly as broad as other hotel giants, like sister company Tingo also only checks for price changes on its own site -- if you spot a lower price for the same hotel on a rival site, you won't receive any credit. (Hotels often charge different rates on different sites, even for the same room on the same night.)

It is yet to be seen how hotels will react to this “cancel and rebook” system.

Some hotels may shun Tingo for making it too easy for customers to cut their hotel costs, though others may like the service if it generates more business than rival sites.

Since 2009, travel agency Orbitz has touted its Price Assurance program for reservations that are prepaid through its site. If another Orbitz customer books the same type of room for the same night's stay for less than what you paid, the agency cuts you a cheque for the difference (up to $500 per hotel room). The site says it issues such cheques every seven seconds on average.

Travelocity has a similar price guarantee. If you book nearly any hotel with Travelocity and then spot it for cheaper elsewhere online, up to a day before arrival, you can request a refund for the price difference and receive an additional $50 discount toward future travel.

Travellers who prefer the do-it-yourself approach should try Yapta, a free travel tracking service. Since 2009, the site has offered e-mail updates on rate changes at more than 110,000 hotels worldwide. Choose a property and the system will alert you whenever the hotel cuts its price. Using Yapta to cancel and rebook only makes sense, though, if a hotel allows you to cancel for free. Some hotels charge rebooking and cancellation fees, and in those cases you'll have to do the math to see if it would pay to rebook.

Yapta, (available on Kayak and on its own site), also has a price-tracking service for plane tickets. Many fliers don't realise that major US airlines provide refunds if the price of an identical type of ticket (meaning: the exact flight, with a comparable seat) goes down after booking directly with the airline. Of course, airlines won't tell you when your fare changes, so travellers rarely see the money. But Yapta will e-mail, tweet or text (via its optional smart phone app) if your fare drops more than the standard rebooking fee charged by US airlines, which ranges from $75 to $150. They also give instruction on how to fill out the money-back paperwork with your airline. Signing up for the service is as easy as a single click, and MasterCard users can sign up via a dedicated webpage, which means that any flight purchase will be enrolled in Yapta's price tracking by default. Note: Yapta doesn't sell tickets, and it only works on tickets bought directly from airline sites; tickets bought through an online travel agency like Priceline can't qualify for refunds from the airlines.

On 22 March, TripAlertz, a free membership-based site for travel, rolled out its own "lowest fares guarantee". After booking a domestic or international plane ticket through TripAlertz, the site will match any cheaper flight that might pop up “within four hours” on any other online travel site that posts publicly available fares, such as Expedia or an airline's own website. The site also pledges to pay customers double the difference, up to $100 per ticket, with a maximum of a $400 bonus per household or group of associated parties.

Orbitz's Price Assurance program says that is another one of its customers nabs a rate that's at least $5 cheaper for the same flight, the agency automatically mails a refund cheque of up to $250.

Auto rentals
Since 2010, booking engine Autoslash has done for car hire what the above services have done for lodging and air travel. Book a rental through the site, and Autoslash will trawl the Web for sales, rebooking your reservation at any better rate that may come along. The site says it finds cheaper deals upwards of 85% of the time.

Among all of these price-monitoring services, Autoslash deserves the highest praise for its flexibility. In addition to tracking prices on cars booked through Autoslash, the site allows travellers to track the prices of a car hire independently booked on a rental agency's website. Just type in your confirmation number and Autoslash will notify you when it has found a lower rate, which you'll have to rebook on your own.

On the down side, Avis, Budget, National, and Alamo do not make their cars available through Autoslash, somewhat narrowing the options. Even so, the site still delivers savings with Hertz, Thrifty and Fox for domestic US trips, with a claimed average savings of about 25% off for travellers booking a few weeks ahead. Neither AutoSlash nor the company you first booked with charges a fee for any of these services.

Holiday Packages
Travelocity has a rate-protection program similar to Orbitz's but geared toward air/hotel and car/hotel packages. Its guarantee enables you to apply for refunds of up to $500 if it, or another travel website, offers the identical package for less, though you'll have to submit proof of the lower price, which can be a time-consuming process. In a similar policy, Expedia will also refund the price difference if a traveller finds a cheaper price for the equivalent package, but the agency only acknowledges sales found within 24 hours after booking.

Sean O'Neill is the tech travel columnist for BBC Travel 

Correction: A previous version of this article incorrectly named Tingo's parent company. It has been corrected.