So, I've been doing research on "easy ways to make money if you're a magi" - the obvious one being salt, but unfortunately that gets into the Rule of Silver. However, in skimming through TME, it seems that while producing a product using magic counts against the 2mp/magi/tribunal limit, TRANSPORTING a product with magic doesn't.
And so I got to thinking - what's a resource that's easily available in one part of a tribunal that's scarce in another? And the answer is obviously (if you're read the title of this post) - Ice. So, I got to researching the ice trade; according to the wikihttps://en.wikipedia.org/wiki/Ice_trade, the ice trade was primarily a 19th century invention - this seems to be a consequence of improved transportation (shipping and rail), as well as other factors, that allowed such an industry to be created. However, storing ice for later use can be traced back in Europe to the 1600's, as well as ice pits in the late roman Empire. (https://en.wikipedia.org/wiki/Ice_house_(building)). So the concept itself isn't unknown - a hermetic magi could reasonably have heard of the concept.
The question then, is - how much is it worth? Ultimately, I suppose the response may very well be "how much time are you willing to put into it" and "what kind of magic are you using to transport it" and ultimately "eh, it's just a minor or major income source, anyway."
In context: the transportation mechanism is a flying obelisk of granite - with enough lifting power to fly ~40 mph, and carry ~1 ton (2,000 lbs) of cargo. The game is set in the Provencal tribunal, in the Massif Central region. Which, in looking at the pictures, doesn't seem to have year-round snow. So, the actual solution would be either to build ice-pits or ice-houses and store them at high altitudes during the winter (how hot does the Massif Central get during the summer?), and load up the sled during the summer to sell the ice in lower areas - or alternately, to just fly over to the Alps or Pyranees, and get it there.
There is of course the second concern of "who do you sell a ton of ice to in the middle of a Mediterranean summer?" - at this scale, I'm guessing the answer is "nobility, for cooling off fancy drinks" - there's not nearly enough scale here to use it for food preservation (and besides, circular wards do a better job of that). And yes, there are the associated stories of interacting with nobility, and getting a supply chain set up, and figuring out how to get the ice the last five miles (ie, land the sled outside a major city at night, load it up in a cart insulated with hay, and haul it to the seller the next day.) And of course the consequences of flying around Europe on a granite obelisk. That's all understood.
So I suppose the real question is this: how much ice do you need to sell in order to get a minor income source? Salt is the high bar for effort/reward - 1 trivial spell, and you've got 60+ mp of product (which, again - you're stuck with the Rule of Silver, so you can't actually sell all of it.). I imagine that ice probably isn't quite in that range, but still pretty pricy stuff. But are we talking 1 mp per ton? PRetty good money for a few day's work, but it only runs during the summer - and could you keep up those prices?