If you've fully integrated magic as part of the setting, then armies will certainly have spellcasters attached, and fireball will be the equivalent or artillery. What about a guano road, which this commodity only produced in a specific, huge bat cave or a bird-covered island ? Many political players would be sure to have strategic reserves of the thing, and if you rule that guano has an expiration date for fireballing purpose they'd need to restock on a regular basis. I'd classify that as semi-silly only.If you want a more mundane choice, remember that transportation costs makes it infeasible to haul cheap goods over long distance. So you'd want to select a high-value commodity that is very easy to come by at one point of your setting and difficult to get everywhere else. The most traded goods from the Baltic were furs and amber for this reason.
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