Have you ever rented a car and then asked for an upgrade when you get to the rental car counter? You may get a free upgrade if cars are available. It doesn’t hurt to ask.
Do you have a membership in AAA, AARP, a frequent flyer plan, or some other reward program? If so, check out their website to see if renting a car through them is a better and cheaper option. Even if you don’t book through them, ask about discounts at the counter for members of your program.
Check the car for dents, scuff marks, and anything else that could be looked at as damage to the vehicle. Make sure everything works. Take photos of anything that looks like it could be potential damage. Renting in France, damages exist on many cars. It’s just a way of life. It doesn’t hurt to protect yourself with photos.
When you return your rental car, wait until the agent checks it over and hands you a final bill.
Many car rental agreements in the US don’t let you drive to Mexico and sometimes not even to Canada. Your car insurance may not cover international rentals, either. Your credit card company may offer some protection but maybe not enough. It would be a good idea to purchase insurance specially for either Mexico or Canada from the rental place.