Vamos en Coche

Normally it is allowed, you can check here:-

To view the CROSS BORDER or FERRY terms and conditions. At STEP2 of our 3 step online booking process, alongside the car photo of your choice, there is a link entitled “IMPORTANT INFORMATION”. Clicking on this link will open an additional smaller window, with the terms and conditions appertaining to that particular car/supplier. Choose then the CAR tab and look under TRAVEL RESTRICTIONS Try a few different suppliers as each supplier/vehicle type may have a different cross border rule.

I had a look for you, all suppliers allow crossborder, except Rhodium.

Some apply a fee for cross border, some don't. Check yourself which is the best combination (rental rate / crossborder fee).

No, you can mention it on arrival unless they say under "cross border restrictions" you have to contact them upfront.