All you need to do to get a quote for the best deal available on airport parking is to select your departure airport from the the drop down list and then enter the dates and times for departure and return. Click on the submit button and you will get a complete list of all the available airport parking options for the dates and times entered for your departure airport. The list will also include information such as the transfer time from the parking location to the airport and the frequency of transfers, sometimes it is not just about price but service as well.
There is also an information button which will give you additional details about the various services offered. These include directions to the car park, arrival and return procedures, security and facilities for the disabled.
You can then make your choice from the list presented by clicking on the 'Book Now' button next to the service provider of choice. When you click on the 'Book Now' button you will be taken to the website of the actual service provider where you can make your booking.
So in fact your cheap airport parking is actually done with the service provider and not through a third party and you will have all the information you need to make a decision before committing to a purchase.