AI just made my first wedding cake for my nephew. My buttercream was as smooth as can be when I left home. When I delivered the cake 45 minutes away, it was bumpy and lumpy and starting to crack. What am I doing wrong? The temperature here is in the 50's, humidity is low. I has crumb coated and applied a final coat. It wasn't very thick.

I had finished decorating it the day before and stored it in a cake carrier overnight in my basement which is about 60 degrees. Since my flowers were made in fondant, I didn't want to refrigerate it.

I had used the standard American buttercream recipe with Target brand shortening, she wanted the frosting to be pure white.

Would I have better luck with Swiss buttercream? I have had his happen before. It is so frustrating when you have went to so much work and then this happens.

Thanks for any advise you can give me. I don't have a business, I just do the for friends and family as a hobby.

Crusting BCs will crack & shift as a cake is jostled.  The cake is more flexible than the icing. 

A bit more moisture in the BC, stiffer cake boards (doubled/tripled up cardboards or 3/16th inch foamcore), and a layer of foam (memory foam is best) under the box in the car can all help minimize this problem.