Carnival is not doing post-cruise testing on the ship, even for a fee. In Barcelona they were offering testing at the port, after getting luggage and passing through customs/immigration, for a “convenience fee” of $100. Another option is testing at the airport, which I did at BCN. With an advance appointment the testing went quickly, and I was there only about 20 minutes start to finish. People without appointments were waiting 2 hours and some missed their flights. I didn’t want to do a proctored test because the internet on the ship was “iffy.”