Please note, and there are plenty of comments on this on the forum, that it is ilegal to offer a collection service for airports in Spain for payment and users of such services will not be insured while in the vehicle
You cannot get hire and reward insurance in Spain
Yes you will save money as long as something doesn´t happen and if the Police catch the driver they may take away the vehicle and crush it
Please see local newspapers as in the Mazzaron area recently Police mounted a campaign of people offering this service , be safe please and use a taxi or specialised airport collection company running insured vehicles