Originally posted by Turnbull:
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. 
Turnbull,
But wasn't Carlo whacked at the mall? As I remember as soon as he got into the car at the mall he was garroted.
By the way, I believe at that time plane tickets were refundable, so if there really was a plane ticket in the envelope, they could have gotten their money back, if they wanted. Don't forget they left the car that they whacked Paulie in, so a plane ticket would be a "mere bag of shells".
