Michael obviously didn't want Carlo killed at the mall, where Connie or another "civilian" might see his body being hauled out. The ticket was a ruse to make Carlo think Michael really was going to spare his life. That way, Carlo would relax and get into the car with Clemenza, believing he was being driven to the airport. Killing him in the car was a good way to mask his murder.
Some people believe that Michael asked Carlo to confess because he wasn't certain, at the very end, that Carlo really ratted out Sonny, until he heard it from Carlo's lips. I don't believe that, but if so, Michael may have bought the ticket in the off-chance that Carlo was innocent.