We should remember that the cars that are rented in airports are subject to additional costs or airport taxes ( as it is stated in your voucher).
When you take your car from another place instead of the airport,  you save money.
However, you should get to the Car Rental Office by yourselves.
The quality of the service is also important; If you have already been satisfied with a rental company for their quality and activity, it would be a good idea to work with them again.
Relatively smaller and independent suppliers may look attractive to you with their lower prices, unique, different and egzotic car models.
Make sure about what kind of road assistance they can provide you.
In this case you don't pay all costs from your pocket when your car does not work.
Try to arrange your self in working hours of the supplier company.
Some companies are not opened for 24 hours.
Book your car at least one week before your pick-up rental date.
The sooner is better, just like booking flight tickets.
Early reservation can provide price advantage ( except for the high season ).
Moreover, some rental companies can offer you discount with online booking.
Get information about the cancellation policy of the supplier.
Especially be aware of any cancellation fees and you should learn when to inform supplier company at latest, not to pay for fine.
Car Rental companies would offer you several choices in general.
They can request  you to pay cash for the fuel in advance and give you the car without fuel.
You should select your car depending on the prices, car models and services offered.
If you are a frequent traveller, you should know that big car companies provide repeat customers with discounts, wide selection of car models and faster service options.
For the people who travel rarely, it would be better idea to compare car rental prices according to the dates, travel destination of the car (mini, family, SUV and others..
) to be hired.