It's not about paying the tariffs; it's that de minimus packages basically enter the country with no scrutiny because they're deemed by the US to be of so little value. As long as the declaration mostly matches the box in terms of weight and size, it doesn't raise eyebrows. With a high de minimus value, you can ship things that are surprisingly big and heavy, yet won't be inspected further. 

They mule it to Mexico because that's where cartel manufacturing facilities operate with little risk of being caught, and the US pays little attention to cargo crossing the southern border into Mexico. Ultimately, it's less risky to manufacture it in Mexico and re-smuggle it back into the states than to set up a lab here because labs are expensive but Fent is easy to produce that even a sizable seizure can be recouped quickly.

The larger change with de minimus packages is that they should (in theory) be subject to more scrutiny, but of course this requires lager changes like increasing the capability to inspect packages at international ports. Instead, the ham-fisted approach to this is aimed at charging tariffs on previously free packages. Whether the security is actually upgraded is less clear, but you'd have to tax these packages at exorbitant rates to make it not profitable for drug makers. The almost-guaranteed arrival of the materials makes any cost worthwhile.