I have to do a similar thing, just for curiosity what was written in that letter?
I'm not sure of the exact details, if I recall correctly it said it was going to be disposed of it within a certain amount of days if I didn't contact them back saying it was marked mistakingly or whatever. I ignored the letter and nothing happened after, was probably 7 yrs ago.
I'm not sure if it was a suicide related thing that you ordered, but after doing some reading on Reddit some people say that these "love letters" may be sent from scammer dealers also who don't send any of the actual purchased product to make it seem like it was seized... but the wellness checks to the door that were done to users here who bought from the busted SN plug I'm pretty sure we're just from the addresses being breached after his arrest. I could be wrong though, I don't know all the info.
There's always tryhard cops doing petty stuff though and anything illegal have to think of the worst case scenario, so if they ever show up act confused and deny ever ordering anything if it happens. Sad world we're in...