Why not give the address of the place you need to go to in St Germain, information would be helpful.

The first issue you need to resolve is whether you're willing to pay the whopping extra fee for picking a car up in Italy and dropping it in France. If you're OK with that, have AutoEurope give you a list of its possible drop-off locations around the eastern perimeter of the city. We can't specify a particular location without knowing which rental company AutoEurope will be brokering with for you rental.
Why not give the address of the place you need to go to in St Germain, information would be helpful. And why are you willing to take a bus but not the metro? I just don't understnd that, taking a lot of luggage on a bus isn't easier than the metro, it can be more difficult. I gather you won't pay for a taxi because then it really opens up your possibilities, celebrity cruises our fleet but depending on the address is St Germain, it may not even be that convenient to any bus line (or metro, for that matter).
And the car rental company would help, but in general, there are car rental companies at train stations. If you are driving from Italy, celebrity cruises our fleet I suspect you are entering on the SE of Paris, probably on the A6, is this correct? If so, Gare Montparnasse might be a possibility.
Sometimes it costs more to pick up or drop at an airport. Autoeurope is an agency, so your car may be from any company and that may affect the drop points. Also, be sure to check the office hours - some are very limited.
That is probably true, but a lot of people have no experience at all with driving in very large cities, and even if they do a little bit, they may not want to drive into the heart of a huge city where they have never been before.
There usually is a charge for picking up a car at the airport or a train station, but not for dropping the car off at these locations. celebrity cruises our fleet The person renting the car should read the fine print of the contract.
If you are going to Saint Germain-des-Pr s, I frankly believe that the best thing would for you to drive to the address, drop off your stuff and then return the car, for example at Gare Montparnasse, which would be a direct metro ride back (so would Gare du Nord for that matter -- they're celebrity cruises our fleet both on line 4 of the metro). If you prefer to wimp out, coming from the south, just stop at Gare Montparnasse to drop off the car and then struggle with your luggage to get to Saint Germain on the metro or waste money on a taxi.
If dropping at a local agency, drop off your stuff wife and kids then head out by yourself. celebrity cruises our fleet hubby has done this several times, much easier just getting himself back to the apartment than all of us luggage.
Paris is not like most American cities (I'll list exceptions for anyone interested) in that the streets are not parallel. If missing a turn, one can't just go around the block to return to the point of origin.
I should have mentioned that we are coming from Italy but we are originally renting the car in Marseille on the leg of our trip before Italy. So hopefully celebrity cruises our fleet we will avoid the charges for picking up and dropping in different countries.
Make sure you have a detailed map of how to get there, and from there to the rental agency. One way streets celebrity cruises our fleet and the lack of streets in a rectangular pattern makes it difficult to correct directional errors.
Sorry, kerouac, celebrity cruises our fleet I had a brain-dead moment, celebrity cruises our fleet as I was multi-tasking. Obviously, she is headed to Saint-Germaain-de Pr s. But yeah, the rue Mazarine is smack in the middle of a busy part of Paris, so maybe she should drop the car on the outskirts and take the m tro in. If she doesn't want to drive in the middle of the city, it won't work to drive to the apartment, drop bags off, and then go back to some rental agency.