You are responsible for the parking fees of the car throughout your rental period. If you end your rental before the scheduled end time, you remain responsible for the parking fees until the originally planned end of the rental.
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Parking for electric vehicles is free only within inner Paris (Paris intramuros).
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Be careful: free parking does not apply in neighboring cities and suburbs (Neuilly-sur-Seine, Levallois-Perret, Montrouge, Boulogne-Billancourt, etc.). Outside Paris city limits, parking must be paid for, or you risk penalties ranging from a class 4 fine to vehicle impoundment.

You can find here details concerning consequences of unauthorized parking.