score:8
It’s clearly not possible. Fare gates are equipped with only two readers:
For the first type of tickets, there’s no choice but a physical paper ticket bought from a machine or from a sales window.
For the second type, you currently need a Navigo contactless pass, and then load it with a day-pass.
They are currently working on extending Navigo passes to other types of tickets (using a different type of pass), but that doesn’t change the problem.
There are experiments to load passes on compatible phones, but this is very very restrictive, definitely not for tourists.
There is no support for credit card contactless use (they have talked about it but it’s still not effective).
So, no, I’m not aware of any way to have anything but a physical ticket at the moment.
Upvote:3
I have just found someone asking exactly the same question back in 2016:
Q: If arriving at CDG airport with family, heading for Gare du Nord, is it possible to buy RER single-trip tickets in advance to save delays and currency problems at the airport?
A: Only multi-use Paris Metro/RER passes can be purchased online ahead of time. Single use RER tickets such as for the RER B from CDG to Paris, can only be purchased at ticket windows, offices and vending machines within Paris.
Source: https://parisbytrain.com/question/can-one-buy-rer-tickets-online/
Now, three years later, the answer still seems to be "it is impossible".