I'm making a 2 tier cake with a fondant elephant  with a balloon on top. I have to deliver it the night before the party because of my work schedule.  Do I deliver it put together or do I wait till I get there to put elephant/ balloon on top?  It will still need to be transported to the location by the buyer so I thought about just letting them put the pieces on.  

I would probably leave the topper off and let the customer place it on top once they get it to the final location.  Just make sure that placing it on the cake is super easy for the customer.

I agree with Freckles0829, let the customer add the topper once at the venue.  I always put my toppers on a separate board so they can easily be placed and removed.  Most people like to keep them, so it makes a nice base, as well.