I iced and decorated my cake last night with buttercream. I need to add my royal icing decorations but am not sure if I can put the cake back in the fridge after I put the royal icing decorations on the cake? (My client can't pick it up until tomorrow night and I work so I need to do the decorations tonight so it will be ready for pick up tomorrow night.) Please advise what I can do? Thank you so much.

RI and fridges are not friends
I would leave the deocrations off and place them just before she collects or alternativley leave the cake out of the fridge if possible

Unless there's something perishable in the cake (custard filling, or something like that), there's no real reason to put it in the fridge. A cake that's been crumb-coated and iced (or covered in fondant) is pretty much sealed against drying out and will stay fresh at room temperature for three or four days. And let me just say that room temperature in my kitchen is 90° and above most of the time, so heat is not a particular problem, either.

Personally I wouldn't put them on, but I live in an area that 5 seconds after I pull something out of the fridge it is covered in condensation. Moisture and royal don't really get along.