So you book a car with Hertz and you plug your frequent flyer number into the space provided. Rent cars, get free flights – no brainer, right?

Not really.

Did you know that when you try to collect points, what you're really doing is buying miles?

Rental car companies haven't exactly gone out of their way to tell you this, but it's right there in the fine print, where you probably aren't looking. All of the major agencies now levy a daily surcharge for miles collection.

They insist this isn't a big deal – after all, they say, it costs them money to participate in these programs. Think of them as being kind of like that guy in the corner store who takes credit cards, even though it costs him. He does it because people get mad if he won't; he doesn't want to get a reputation for being that guy who only takes cash.

Generally, though, Mr. Corner Store will either eat the cost of credit card processing, or build it into his prices.

The rental car companies, so eager to give you the lowest base rate they can possibly offer, prefer to just add everything in on the back end.

So maybe you're thinking that a small fee doesn't sound like a bad thing. And maybe you're right – sometimes it is small.

For example, Hertz charges members of United's Mileage Plus program just 6 or so cents per day, based on the federal tax it pays on the miles it purchases from United.

That may not be such a big deal, particularly if you're booking one of those cool mileage deals United runs – say, the 35 percent off + up to 1,500 miles offer they're running through Hertz at the moment.

Plug a JetBlue TrueBlue number into your reservation, however, or your US Airways Dividend Miles number, and, oh, look at that – you're now paying up to $1 per day to take advantage of what you thought was a free benefit.

Sure, they might cap it at $5.25 per rental, but still. What if you only earn 50 miles a day -- you're now paying $0.02 per mile. Does that not seem like a lot? Okay, but consider this: US Airways will sell you miles, any old time you like, for $0.0275.

If you're into buying miles, that's fine, go ahead. Congratulations, you just saved $0.0075 per mile!

But don't be fooled into thinking you just landed a freebie, because you didn't. (And don't look at your price quote at time of booking for any of this information, because you won't see it – none of this shows up until you're actually at the rental car, in a hurry to get out of there and less likely to read the fine print.)

All in, sure it's not a lot of money. But with all the other taxes, fees and hidden charges that come along with renting a car these days, you really need to be aware of how quickly they can add up.

How quickly that happens depend on who you're renting your car from. Let's take a closer look – via a handy chart -- at how each agency deals with the miles question. One thing's for certain, when we look back a few years – not only are the fees here to stay; they're climbing.

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