Thank you all. Just to clarify more: we have plenty of time, i.e. from 5 pm to 12 midnight to reach ORY. We will be staying near Ledru Rolling. We plan to walk to Sully Morland, take Metro 7 from there to Porte de Choisy. Then get bus 183 to ORY Sud. As Kerouac said, we can enjoy sitting in the Metro. We have small cabin bags with wheels, 1 each. We did it earlier in 2010 with Metro+OrlyVal. Probably we didn't know the option of Bus No.183. If there is snow, of course we have option to take taxi. We searched online taxi booking which quotes 70 euros approx. and that was our reason to calculate saving.

Didn't know there was the four of you. Wait until after 8 or so and you'l be ok.

Flat rates are good for 4 passengers and all their luggage.

Could go to Ponte de Choisy with regular t+ ticket and from there got bus 183 to Orly sud at 2 euros ticket on board. Had to wait for 20 minutes for 183. After getting down from bus, there was a shuttle available immediately to Orly west terminal. It was quiet and undisturbed night at airport hall2. Overall good experience. Saved on hotel and taxi.