I have always used Luzcar. They do not normally charge for taking cars over the border into South Spain.
If you read back on this blog you will find several posts about them.
I have taken their cars to Seville,(indeed the last time was a couple of weeks ago,) and that Southern Region of Andalucia, with no problem.
Their prices may appear a little higher than others but their prices are inclusive, including 2nd driver, except for optionals such as additional personal insurance cover.
Make sure that you tell them you are going over into Spain.
Their site is:
http://www.luzcar.com
