For instance, cocaine costs ~2k/kg in backcountry Columbia, but ~20k/kg in US cities [0] (at least at the time the article was researched). And when your transport costs dominate that much, almost every way to reduce volume and weight is justified.
0: https://www.businessinsider.com/from-colombia-to-new-york-ci...
The total cost of risk-free (i.e. fully insured) delivery of a truckful of goods is trivial compared to even quite cheap products; while that briefcase would pretty much have to double in value after that long trip to make the transport profitable.
Fentanyl solution for injection that hospitals get is a lot less potent than the hydromorphone solutions for injection that they can stock.
(50mcg/mL vs 10, 20, 50 or 100mg/mL).
As for the actual dosing... fentanyl is a lot more forgiving than morphine or hydromorphone, as measured by therapeutic index.
The ODs are mainly because people don’t know what they are buying.