The list of things customs look for while checking the incoming packages.

[Charon]

I have heard that the medicines written up in Letters of Interdiction only include one med. And, they often get that one med wrong.

If you watch a customs and border patrol series, They seemingly use the dogs, xray, then, if and when they open a package, the substance is put in a glass jar. If it turns one color, it is XX. If it turns another color it is XXX.

So, they may be investing in manpower more than the chemical screening process and its accuracy, unless a full blown prosecution is planned.