I made a two tier Dulce De Leche cake with Swiss merengue buttercream and put 5 dowel rods in the bottom and one going through the center of the top tier all the way through even the cake board in the bottom. It was a 7 inch over a 9inch cake. I had been decorating the cake in an unconditioned environment unfortunately but still no matter how I moved the cake it seemed very sturdy, even when I patted the top tier to make sure the sprinkles were well in. Once I put it into the box I kept the cake in the fridge for two hours before the customer picked up and the box was even cold when I gave it to them. An hour later I get a call that the cake fell apart. I just don’t know whether to believe my filling could have melted or did it take a hit in their car. I’ve never had this happen before but I can say I’ve had a cake filling melt which was an experimental cak and filling had splurged out but this one doesn’t have that. 

...............two tier Dulce De Leche cake with Swiss merengue buttercream .........

Never worked with either of those mediumns but DDL cakes are *VERY!* moist and unstable from what I understand.  I refused to make them for few times I was asked to.   It looks a bit like it might have been dropped or maybe driven by a careless driver {going around corners on two wheels ;) }

so sorry that happened -- it looks like the layers are sliding apart -- if you put the cake in the fridge in that box then it insulated the cake from the cold air of the fridge for two hours -- maybe that's why it slid apart -- so so sorry