Fist of all, if you are considering a rental car post-cruise, Carnival transfers can be purchased one-way. I believe, however, that Carnival's shuttle service does not begin until after 10 AM - leaving you hanging around the airport for awhile. There are a number of reliable transfer and shuttle services available from the airport (go to http://www.portcanaveral.org
for the list) that you can use that offer 24/7 personal service, that (depending on number of people in your party) may be cheaper AND certainly more convenient. As an example, I can recommend Art's Shuttle. The one-way rate for 1-3 passengers is $60, for 4 it is $70.
Hope this helps,