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.