I have used Tiffany Towncar on three seperate cruises and have never had a problem with them. On the last trip I was thinking about using Happy Limo because they were cheaper, but with all the problems that were discussed on this board I changed my reservation to Tiffany and don't regret the decision at all. On each of the trips, the drivers were at the pickup location early and made a stop at a drugstore without any problems.
I would highly recommend Tiffany and if you call you should be able to get a reservation even with such short notice.