Originally Posted by Sheikh Yerbooty

Nope, Sixt can't charge you for the fine. They might very well charge you a fee for the process of handing over your personal details to the Guarda Civil, but they've no legal leg to stand on when it comes to the speeding fine - that's strictly a matter between you and the Spanish authorities.

This from a bloke who's received numerous letters from various German constabularies, including the most memorable one which had a piccy of self going through a 120 zone on the Autobahn at a recorded speed of 218 km/h in a Sixt owned vehicle; a BMW 630i does pick up the pace rather nicely. The local law enforcement agency invited me over for a little chat in front of a judge, which I chose to ignore. Haven't heard a word since, and I've rented many a car with Sixt between then and now.

So unless you physically get pulled by the plod next time you're in Spain, bin the letters from the law.