WOW, man! Not really any crimes committed, but you go with that if you like. Why no crime (besides the fact that you're pretty far off on what happened related to charges you make)? Customs and the USPS are governed by FEDERAL US laws. Contrary to what you say, NOTHING here is "federally legal dr approved". MJ is still illegal, and a Schedule 1 drug by US law (same as cocaine, heroin, LSD and many other things), no matter what your state or Dr. says. Yes, believe it or not, the DEA (or FBI or even state or local officers with guidance/direction from Fed Authorities) can still come take all your meds and paraphernalia and hit you with with Federal charges - even if you live in CO, WA, CA or anywhere else. They usually don't - and haven't been doing it much for years - but that doesn't mean they can't or won't. And, think when they have done so in recent years they usually go after dispensaries.
All that aside, it sounds like it may be bogus to me. Don't recall reading about this here for more than a few years, tho IIRC it had happened once or twice way back in this thread. Did you get an actual on paper letter? Is it on US Customs letterhead? How would a scammer grab your piece? And, if it was someone local who snagged it off your doorstep, why write a letter? I guess a vendor could scam you with a set up like this, but doesn't sound like a Chinese one. Way more work than necessary when they could just say they sent it, and never did. Not hard at all to fake shipping - and on DHGate, they don't get your payment until you approve/release funds - so what would be the point?