Something I found when pricing my Christmas trip this year is that renting the car at the airport isn't necessarily the cheapest place to get it.
I plan to take Disney's Magical Express from and to the airport, then pick up my car at the Hertz counter at the Shades of Green Resort. Using the
AAA web site, I was able to find a rate at Shades of Green that saved me almost $300 off the airport cost. (I'm also saving a day on the rental because I'm turning the car back in the day before our departure -- we're there for 8 nights, so I'd have had to pay an extra day fee if using it to get to the airport the next morning.)
It's not quite as convenient, but it really isn't too bad -- especially for that kind of savings. I'll take DME to get to my resort, then Disney transportation to get to the Polynesian, then I'll walk across the street to SOG to pick up the car.
Other rental car locations that can be reached with Disney transportation and a little walking are: National/Alamo at the Dolphin Hotel or Magic Kingdom Car Care Center; Dollar at the Regal Sun Resort (right across the street from Downtown Disney), and
Avis at the Hilton (also right across from DTD).
If you're an AAA member, check the rental car prices through the AAA web site instead of the rental car's web site. They'll push you toward Hertz, but you may get a better deal through them than you can find on your own.
As far as the pre-paid gas, this is a personal preference. You need to decide how much gas you're going to use on your vacation. If you think you'll empty most of the tank and be able to turn it back to them on "E", then the pre-pay option is the way to go. However, you certainly don't want to buy a whole tank of gas, then turn it back to them unused. (There's no refund for gas you didn't use.) The biggest up-side to having a pre-paid tank is that you don't have to spend the time to find a gas station on your way back to the airport to top the car off. Just about any rental company offers the pre-paid tank.
You can rent car seats from just about any rental car company, too. There should be a place to check that you need them on the web site when you make the reservation.
One other thing to do before you get to the rental counter is call your insurance company. Ask them if the policy you have for your car covers you when you're driving a rental car. Also, if you're going to pay for the rental car with a credit card, you can call the credit card company/bank and ask them what coverage you get automatically through them when paying with that card. If you have several cards, call each to find out which gives you the best coverage. The cost of insurance from the rental car company is very high, and won't be part of the estimate you get online. However, in many cases, you don't need it any way, because you already have coverage.
Another thing that costs more and isn't reflected in your online bid is additional drivers. This can be as much as $10-$15 per day, depending on the company. If possible, pick a person who will do all the driving, and have that person rent the car. If you absolutely need the flexibility of having more than one person driving, be sure to register the additional driver. (It'll cost you, but you really, really don't want to be in an accident if an unregistered driver is behind the wheel.)
As mentioned by the previous poster, the online bid is never the price you're going to pay -- even if you don't add extras like pre-paid gas, extra drivers, or insurance. This figure is usually the "base rate." After they add the airport or resort fees, taxes, and other additional fees, the final cost will wind up being a good 20%-30% higher than the bid that came up in your internet search. For example, on my booking, the Base rate is $341.60. In smaller print further down on the screen, it tells me my approximate total charges will be $419.76.
