You may take the Hertz car into Spain (as long as it's not into the Spanish enclaves of Ceuta and Melilla).
at step 3 (choose extras)
Afer the Available Extras section there is a link for
Rental Qualifications and Requirements
When you click on it, it asks you to choose a topic; I chose driving restrictions to see if there were any restrictions for day trip into Spain with a Hertz car rental from Montpelier, France and received the following information:
"Hertz vehicles may only be taken in the following countries: Andorra, Austria, Belgium, Denmark, Finland, Germany, Gibraltar, Ireland, Italy, Liechtenstein, Luxembourg, Monaco, Netherlands, Norway, Portugal, San Marino, Spain (not including the Spanish enclaves of Ceuta and Melilla), Sweden, Switzerland and the United Kingdom (including Northern Ireland). "
There was also an note specifically for for rentals travelling into Spain that all vehicles are equipped with a security kit (fluorescent jacket and a triangle) free of charge.
This kit must be kept in the car. If both parts are not returned there is a penalty fee of EUR 15.05 (including tax).
Also a note that when you rent a car from France that will be travelling into Spain, you must purchase an extra security kit for EUR 10.03 (including tax). *Note: prices accurate at the time of this post, but may change without notice - always double check fees before renting.
Hope this helps!
International Car Rentals