I would think you would wash the egg cartons first, but maybe that isn't good enough, not sure.
I thought of another thing I failed.. I tried one of the Cake mixes where you use pop. The pin I saw had strawberry cake and sprite, but I mixed it up and used strawberry pop and a white cake. It was such a sticky mess. I did recently try another cake and pop mix that was much better, it was a diet pop with a cake mix. I used a Sugar Free Devils Food cake mix and a can of Diet Cherry Vanilla Dr. Pepper. That one was really good.
I made one of the cake mix and pop concoctions and it turned out great. I thought it was supposed to be able to be done with ANY cake mix and soft drink combo, but I've only tried one or two.