There's a very simple rule in this world of predatory sellers. If they think you are desperate, the price goes up. Just picture the scene. You quickly packed a bag, ran down to the airport and got on the plane. Now you have arrived with a crowd of others and begin queuing at the nearest rental agent's stand in the airport lobby. When you finally get to the counter, there are only a few of the most expensive, top-of-the-range vehicles available to drive away. If you could wait. . . but, obviously, you are in a hurry. Then there are additional fees to cover the paperwork. The price just keeps going up and up. So, to protect yourself against being gouged, you should always prebook as far in advance as possible.
Let's start with a quick summary of the reasons for advance booking. No matter how many times you see the advice, it remains true. Shopping around really does save you money. This site offers you access to multiple car rental companies, all of which are competing for your business. That gives them all an incentive to keep the rental rates low. Now check whether the airline or travel agency is offering a package deal of flight plus cheap car rental (and possibly plus accommodation). If that's the case, you could make a bigger saving instead of buying direct. Or perhaps your credit card or some other affiliate group to which you belong has a promotional offer on car rental. Or perhaps you belong to a loyalty program or your air miles reward program can be used to pay for the car rental.
Then take a further moment or two to check out the off-site rental agencies. Yes, you will have to take the airport shuttle bus or a taxi, which can cost you a few dollars, but often the more distant sites have really cheap car rental offers. If there's any doubt about whether it's going to be easy to get to the location to collect the vehicle or you are worried about finding additional fees, a quick telephone call will usually resolve matters. Remember the rental companies located at airports always have higher rates. Protect yourself and drive away the car you booked, at the time you booked, at reduced rates.