re: Car rental in Europe

To get more explicit about third party bookings, at least the hotel third parties (Crap-pedia, Scameloxity, etc.) create the booking and then pay the hotel with a fixed value pre-paid card. If you want to pay an extra $5 a night for another towel, the hotel literally can't do anything for you. You paid the third party 129.95, the hotel wasn't paid by you. They *can't* do something that costs them 130.05 without the employee getting taken behind the woodshed for helping you, so they won't (rightly so.)

I either book directly through Avis or Europcar, and I never booked cars frequently in the EU/UK/IE, on purpose.

Also, you're talking about 2 cars, and you're going to be paying for parking for each. It may be much cheaper to rent a mini bus from CDG to your hotel than it is to rent two cars and park them for days.