In Italy, when they malfunction, it's always on purpose (for the purpose to get a cash payment and not report it for tax purposes). And don't ask the taxi drivers at FCO to take you to Ostia, where the fare would be only around €20-25. They won't take you. They take only people going to the city center for the full €50.
CDG to Montmartre is a fixed rate -- €50 as Montmartre is on the Right Bank. Make sure you take an official, licensed, legitimate taxi from the taxi line at the terminal. all cabbies are required to take cards but not all do (machine is broken yadda yadda).
The taxi fare from FLR to anywhere in the city center is a flat fare of about 24€ plus 1€ per bag (a bit more at night from 10p to 7am and on Sunday). If you must pay by credit card, ask if the POS machine works before boarding, if they tell you no, then respectfully decline and go to the next cab. Rest assured that at the prospect ofIn summary, while credit card acceptance is increasing in Italy, it is still recommended to carry some cash for smaller establishments and rural areas. Major cities and tourist destinations are more likely to accept credit cards, but it is always wise to have alternatives available and ensure a hassle-free experience during your visit to Italy.