You may find that machines will not take N.A credit cards, but they can be used when a person scans them
(e.g. at a restaurant).

I usually arrive with a few euros in my pocket for just that situation. Go to booth, get ticket (or two) and then get rest of money from ATM in central Paris.

Iregeo,

The machines did not take our cards (even the ones with the chip!), but we were easily able to buy our RER tickets from CDG to Paris at the booth from the delightful young man who indulged our attempt to negotiate in French before letting us know he was fluent in English. The line was negligible. We too were in Paris on a trip that spanned the weekend making the Navigo a bad choice. Instead we bought 5 day, 3 zone Visite passes and I'm glad we did. It rained a lot and the Metro was an easy quick break for our aching feet. I couldn't count how many trips we made on the Metro, the RER, the Funicular and the busses. Whenever we found ourselves too far away, running late or needing a break, we headed for the nearest Metro. We bought individual RER tickets for our excursions outside central Paris. Well worth it. Easy to use. Easy to buy. We still did a LOT of walking! Since it sounds like you're not planning to move around as much as we did, the carnet is ideal!