I can only suppose that Fedex drivers are reliable professionals with a sturdy common sense. They can't be deluded for long, by this kind of staging. A less naive person, let's say an ex Fed (agent), would immediately suspect a prank. But should I be the knowing victim of such a scenario, what could I say that would be adequate to the situation?...'Never Mind'. Never mind the prank, never mind the ridicule, let's play along . The words: 'Never Mind' would come to mind instantly.
I am not quite ready to believe the drivers were so unsuspecting. But, not knowing what to make of that crazy situation, they may have responded out of the wish not to deceive the devotees' expectations. Instinctively, they did what was expected of them. And in turn the prankers interpreted the drivers' words in their wishful way. Stage what you want, who wants to prove exceedingly, proves little.
Of course, provided the proper mind frame and training Fedex drivers could make acceptable swamis. But offered with that possibility, I believe they will prefer to go back to their ordinary life, content to deliver, in time, ordinary things to ordinary people. That's sensible enough.