I am traveling 3 hours by car with a cake completely assembled and if is covered with gumpaste flowers. I plan on having the cake in a stryofoam box with dry ice because the weather is hot and humid. Will this affect my gumpaste flowers? As far as I know dry ice has no humidity, just cold, so they should be fine, right?
Is your vehicle air conditioned?
Dry ice will gather humidity from the air and turn into a puddle.
Put a layer of frozen ice packs in the bottom of the cooler, and put the cake onto that. When you start with a fully chilled cake you have 6 hours time.
Carry the gumpaste flowers in a separate box with some drying crystals if available. Do not put them near any cold item until you get to the venue. Add the gumpaste flowers to the cake at the last minute. Carrying an assembled cake with gumpaste flowers on it is asking for trouble (like being bumped by doors) even on a dry day. Add them onsite and they will remain perfect.