We want to rent a car for a week when we arrive via air in Charlotte in mid-March. Rates seem better at off airport sites. Any recommendations on a rental agency that provides complimentary shuttles for off airport auto rentals?

We had good luck with Alamo (used them twice now)...

We experienced the same high rates when we planned our trip in March. We learned that there were no complimentary shuttles when the agency was located at the airport and that taxi fares were also high. We booked a car from an airport rental at a high rate and then discovered the last minute rates that become available about two weeks before the desired date. When the last minute rate became available at about $100 savings, we cancelled the first booking. I think it was with Alamo.
Budget, Alamo, Payless and National all have Last Minute links on their websites. The locations/airports of the last minute deals vary from agency to agency. Good luck. Hope this works for you.

Is Alamo on or off airport property at CLT?

these folks have come and gone, but I live in charlotte and am renting a car in August at the airport. I got a perfectly acceptable rate there. I think I used Travelocity because Hotwire was so much more expensive.

Gretchen - Are you talking about Alamo? If so, why didn't you go to the Alamo site directly?

No, I got it on Travelocity and it was Enterprise. Our kids are landing and leaving from the airport so that is what we are doing. Have to get to the airport.
I'm going to do what Kibby suggested however and continue to check. There isn't any money up front with these rentals so nothing lost.