So we can agree the shipping isn't make it cost prohibitive. Your point about there being less hay making finding needles easier might be possible (although I don't think it matters, as with our southern border, a higher detection rate just means you need more chance to get the desired total smuggled). I would imagine there would just be more product shipped to Central and South America and then imported like everything else south of the border, cars, trucks, boats planes. "The flow of fentanyl to the United States in the near future will probably continue to be diversified" [1] Production in those places might also increase. Moral of my story is this change in shipping prices will have no long term effect on amount of fentanyl brought into the US; though, it might very well increase the involvement of SotB cartels. If there are buyers, these will be sellers.
[1] https://www.dea.gov/sites/default/files/2020-03/DEA_GOV_DIR-...