Have you tried returning the car in Italy and taking e.g. a train for the final stretch across the French border? Usually one-way rentals in the same country are cheaper than one-way between countries.

You didn't specify what car you were looking at, but e.g. a group C car for the same dates, returning in San Remo is 577€ for those dates, which is 100€ more expensive than returning in Venice. (that's without any FFP discounts)