I'm in Florida (and do a lot of beach wedding cakes) so I definetly know what you mean about humidity and melting icing. I add a teaspoon of powdered egg white to help stabalize the icing. And also make sure to keep the icing as thick as possible, while still making it spreadable. Even then, I deliver the cakes in a van so cold that I have to wear a jacket, set it up at the last possible moment, and do alot of hoping and praying that it will last as long as it needs to.