Just my experience, but I've had an international shipment seized before, and all I received is a letter from from border patrol saying the item in question had been seized because I didn't have a prescription and if I wanted it I needed to produce a prescription and also go through some lengthy process that involved court costs and some other shit.
I also used to work for USPS for several years and a carrier and clerk. We would receive packages all the time that came through partially opened and contained drugs of all kinds that could be clearly seen. If this happened, the package would be seized and disposed of. In the four years of working for the postal service I only saw the postal inspector get involved once when a known drug dealer came into the post office with two packages containing illegal drugs. The DEA and Postal Inspector had already been building a case against this individual and we were advised to hold the packages once they came into the office. From what I understood at the time, unless it is a massive amount of highly regulated harmful substances like fentanyl, meth, heroin, etc., the postal inspector or other federal agencies aren't going to waste resources trying to arrest or prosecute someone for something like the compounds we discuss here. They would also need warrants, court orders, and a lot of other legally necessary shit to fuck with you.
With that being said, deny the shit out of everything. Play dumb. Know your rights and never admit guilt. Go about your daily routine and don't be intimidated. Don't give law enforcement anything.
Feels like there's some more to this if it really happened, but I'm also a moron, so yeah.