I searched and didnt find a good answer so I hope someone can help. 

I made a deer out of modeling chocolate and RKTs and its for a cake on Friday morning. What is the best way to store the deer so that it doesnt melt, droop, etc? 

I stuck in the fridge but then read that will create moisture issues... I have a cake carrier I can put it in and then put in fridge if that would be better. 

Or would it be ok at room temp (a/c is set to 74)

Also.... once I put it on the cake... should the customer keep the cake in the fridge or just a cool/dark area?

When you have a rice treats decoration that's covered in modeling chocolate, it's best to store it at cool room temperature out of sunlight. Pick the coolest spot in your air conditioned room and leave it there. No need to cover it up unless it needs that kind of protection from pets or kids. Once the cake is fully decorated, the 3D topper can remain on the cake while going into the refrigerator.