So recently a customer ordered a mini unicorn cake. I basically made two 6" by 3" cakes and used a 4 inch cookie cutter after cutting them down to layers. I decorated them with buttercream and fondant decorations and froze it for 2 hours before it sat in the fridge overnight. Now it seemed perfectly fine when I put it in the box, no signs of wobbling and the customer picked it up for transport. She lived 30 minutes from me..but when she arrived the cake had fallen. I didn't think it needed more than one central support beam because it was so small but perhaps was the decorations just too heavy for the bottom tier? The cake was a boxed funfetti cake and admittedly moist but I thought it would be fine.   Any advice for future sales? I returned the money to the customer! 

not exactly understanding -- how tall was it? if you used a 4" cookie cutter -- you cut the 6"cake down to 4"? then it was six inches tall? if that is correct -- then you needed more doweling internally --

This could have been the results of an abrupt stop or turn.  Your cake seems to be level and it appears that just a bunch of the heavy swirls disconnected from the cake.  I have not had the experience (mostly because I dont like the rosettes, too much icing for my taste), but from reading other's comments, the weight of the frosting causing it to fall off the cake is pretty common.

I do see one thing that might help you in the future, be sure your cake board touches the box on both sides to insure stability.  Otherwise it can slide around inside. If the box is larger than your cake board, as it is in your picture, cut cardboard (the thick kind, not cereal boxes) to fit around the cakeboard and snuggly against the sides of the box.  It works a charm. You might also want to add a non-skid mat underneath.

All of that being said, I think your customer made a booboo, either driving or picking it up at an awkward angle.

You cake is adorable, by the way!

it's really cute though -- I like the shaggy back of it -- it's well decorated