There's no guarantee you'll get the rental car you reserved
Your rental reservation guarantees a car, of some kind, just not necessarily the one you reserved. This is why sometimes when you reserve a Ford Focus and end up with a Mustang, you aren’t charged extra. The reverse is also true -- if you reserved an SUV and all they have is hatchbacks, sorry Charlie, practice your creative car-packing. The rental company will give you a discount. And if they’re completely out of cars, they’re required to find you one from somewhere, even if it’s another company. But at that point, it may be faster to just rent elsewhere.
You're agreeing to let the rental company track your location
Many rental car companies have GPS in their cars, partly to help you navigate a new city, but also to make sure you’re not illegally taking it on road trips to Tijuana. Your rental agreement often has geographic boundaries for where you can take the car, and if the GPS shows you’ve gone outside those boundaries, they might assess a fee. It’s all in the fine print, but if you’re planning a road trip across state or international lines in a rental car, ask before you sign.
Turning on the toll transponder comes with a big daily fee
Many U.S. cities are now crisscrossed by toll roads, and rental car companies are nice enough to “lend” you a toll transponder to make driving through them easier. However, they often charge a daily flat rate of around $7, on top of whatever tolls you accumulate. So if you see that little box attached to the windshield, and you activate it, you’ve just agreed to at least that daily flat rate. One way around this is buying your own at a grocery store as soon as you land, and activating it before you hit the road.
Often times you also agree to an “administrative charge” of upwards of $15 for the “toll-by-plate” systems some cities use, where cars without transponders are photographed going through toll plazas, and billed for the toll by mail. These fees are even nastier, as you’re hit with a charge for every single toll you drive through, plus the cost of the toll. Same goes for parking tickets if you don’t pay them on time.
Are there other things hidden in the fine print you might want to concern yourself with? Of course, though many instances are rare or deal only with international travel. Starting here will make your trip calmer, and most importantly, keep you from rage-tweeting at an airline who has only to reply, “It was in your contract.”