Need a rental car? Let someone else find you a deal When it comes to most aspects of online travel, it obviously pays to shop around. When it comes to rental cars, however, it may pay to let someone else do it. That's the value proposition at websites like AutoSlash.com and RentalCarMagic.com, which promise to search out the best rates for rental cars so you don't have to. AutoSlash goes even further, tracking users" reservations and alerting them when rates drop so they can rebook and reap the savings. The sites are predicated on the fact that car rental rates fluctuate wildly, rental car companies offer a seemingly endless stream of discounts and promotions - many of which are difficult or time-consuming to uncover - and the fact that there's rarely a penalty for canceling a reservation. For example, on Feb. 2, I booked a midsized car through Dollar Rent A Car for a nine-day trip to Hawaii for $388.59 and then entered my booking information at AutoSlash. On Feb. 12, I received an e-mail from the latter saying they'd found a discounted rate of $307.42, at which point I canceled the first reservation, booked a new one and saved $81.35. "We're not doing anything you couldn't do on your own," said AutoSlash co-founder Jonathan Weinberg. We just take all of the work out of it for you. The service, which is free, works in one of two ways. Users can book elsewhere and input their reservation at AutoSlash or book on-site, which results in a higher level of analysis. All told, the site has processed 100,000 bookings since launching two years ago, said Weinberg, with 85 percent of users being notified of a lower rate. "The earlier you book, the longer time we have to be able to try and find lower rates," he told msnbc.com. Customers who book at least two weeks out are likely to get at least 25 percent savings. In my case, he said the drop was likely a combination of naturally fluctuating rates and a 10 percent Hawaii promotion that, in all honesty, I probably wouldn't have looked for myself. RentalCarMagic also scours the web for discounts and promotions but doesn't automatically track subsequent price drops. To use the service, renters enter their booking information on the company's website, along with potential discount-providers, such as AAA, AARP and American Express. The company works its "magic" and responds via e-mail. For example, in early March, I inputted the information for our Hawaii trip and received an offer three hours later for a car from Alamo Rent A Car for $303.87. The same rental at Alamo.com: $371.65. Savings aside, there are caveats with both services. At RentalCarMagic, searches are free but the company charges $19.95 and up for access to the discounts they've uncovered. Even so, our Hawaii rental would still have netted us a savings of 13 percent. On AutoSlash, discounts are only applied after you book, so posted prices are essentially rack rates, which Weinberg admits has turned some users off. Furthermore, not all rental car companies make their inventory available to the site - Avis, Budget and Enterprise, which also owns Alamo and National, are notable exceptions - so some deals won't show up. It's not surprising, perhaps, that some industry players aren't participating, especially since bookings made on AutoSlash (which are handled by Travelocity) return less to company coffers. Likewise, some companies apparently take issue with the idea of people accessing discounts without doing the heavy lifting required to find them. "The whole point of promotions is that they're narrowly offered to a specific audience or market segment," said consultant Neil Abrams of Abrams Consulting Group Inc. These companies have come up with a model that scours the web for discounts and immediately applies them. They're the vacuum cleaners of these offerings. And they clearly help consumers clean up. Two days after AutoSlash notified me about the $307.42 offer on my Hawaii rental, the company sent another, noting that they'd found an even better price of $289.02. That oughta buy a couple of Mai Tais.