The cupcakes in my profile/avatar pic there where topped with Cream cheese buttercream. I’m pretty sure they where left out after I placed them for quite a few hours. Supposedly, the high sugar/fat content is suppose to make them shelf stable/food-safe longer, but I am always nervous about it and still try to fit them in the fridge. You might give the client the option of risking it or not?
Crystal Memories in the Baking