Chip and signature credit cards

Sometimes they work and sometimes they don't. It seems to depend on the validation protocol of the system and/or your bank.

Personally I would use cash for such a small purchase, but if you must use a credit card (C&S or mag strip) just buy the carnet from a manned ticket window, there's a good chance the card won't work in the machines.

I did succeed to use a US card to buy an RER ticket and a carnet of Paris metro tickets from the machine in Terminal 2 of CDG airport last year. It might have been my magnetic-strip debit card with PIN, since I would have had that handy for withdrawing cash at the airport. A staffed ticket window is more likely to have the necessary internet access for cc approval than are some ticket machines.