Question: I'm going to the Elemental Plane of Fire, and I don't want my ice cream to melt. What's the least expensive and most portable method to keep my ice cream consistently intact?

Answer: 
Seal some [Brown Mold](http://www.d20pfsrd.com/gamemastering/traps-hazards-and-special-terrains/hazards/special-hazards/brown-mold-cr-2/) in a container and put that container inside a larger, insulated container for your ice cream. The cost is the cost of the containers plus the cost of the brown mold (free if you can find it and harvest it).

That's a trick I've been using since 2ndE (the walk in freezers had a ring of warmth on a hook outside it). In this case, the freezer was double walled with the mold sandwiched between. When a friend wanted their own freezer, I had him build a similar room, took some mold from mine, put it in between the walls of his and launched a couple of fireballs into it. It was chilly in no time.
Similar to Léon's answer, though without requiring knowledge of the Prestidigitation spell, you can simply have your players encounter a merchant selling a "bag of colding" (or have them be given one by the quest giver).

I first heard of this concept while watching the Critical Role 5E (née Pathfinder) series of D&D on YouTube/Twitch. During one episode, the party was given a container that acted as a moderate refrigerator by a quest giver, in order to go kill a beast and harvest certain organs, in order to return said organs in exchange for a bounty, with the critical proviso that the organs still had to be fresh (the beast was many days' travel away in each direction).

You could have the questgiver require the container be returned after the quest into the Fire Plane, or have it last only long enough for them to complete their quest before the enchantment wears out. Any combination of criteria that result in 'limited effectiveness' are available to you as DM.