I'm trying to get up the courage to start taking orders for wedding cakes, but this last experience is not helping!

I made a small 2 tier fondant covered cake for an outdoor event.  I always refrigerate to help ensure stability during delivery.  I had been working on this cake almost right up until it was time to leave, so it probably wasn't even as cold as my cakes usually are.  On the 10 minute drive to the event the cake got shiny, but by the time the cake was cut (about 2 hours after delivery) the fondant was positively melty!!  I happened to be attending this event so I heard that it tasted really good, but I was really embarrassed about the state of the fondant.  Thankfully the decorations were minimal and mostly made out of chocolate (ignore the wiggly letters, allergic reaction+benadryl+caffeine makes for very shaky hands!), but I keep thinking about what a disaster that could be if it's a wedding cake with gumpaste flowers or intricate designs.  

If it helps, the fondant is homemade marshmallow fondant.

Is there any way to avoid this other than not refrigerating the cake?  How do you handle outdoor weddings in humidity? How do you ensure your decorations don't absorb the humidity?

Outdoor cakes are going to soften up and be melty if it's humid. Doesn't matter if it's buttercream or fondant. The fondant is the reason it didn't end up with the icing sliding off the cake, so that's a plus.

I think that most cakes that are at outdoor receptions don't look like we think they do after they've been sitting outside for a while. We don't usually see the evidence because we don't go to the party, but you happened to be there as a witness. When you see a photo of a wedding cake at an outdoor reception being cut and it looks perfect, chances are that photo was touched up by the photographer to remove sags and humidity marks. I've seen some nasty wedding cakes that looked great in the professional photos. That would be thanks to the retouching, not to the decorator.

So basically as long as I warn the couple about what humidity can (and probably will) do to a cake I am good to go?  Clearly there are some designs that will work better than others in humidity and I would certainly steer them away from those that won't handle it as well.  I do tend to stress over the details a bit too much!  Thanks for your reply!

Right, there's no way that you can control the laws of chemistry and physics, and sugar attracts moisture to it based on its chemical composition. If you put sugar outside on a humid day it will attract moisture and absorb it. What happens after that will be anyone's guess.