Welcome, pretty much all mushroom growers run out of clients because it's not like smoking weed or meth, where you do it multiple times a day, most people try it 1-10 times in their life.
UNLESS "they are on to you", it shouldn't be too tough. You're an amateur mycoogisy, so you understand a sanitary technique, stick close to that, don't put bare hands on anything. Vacuum seal bone dry mushrooms. put that in another bag. PUT THAT In another bag. AT least one or both of those bags needs to be a solid color, preferably black, sonthat a person can't see what's in it without destroying it. Finally, the cardboard outside package. Could be a small flat rate box. You can buy postage with crypto and anonymously. Don't use excess tape on it or make it seem different from all the other mail.
Optionally, some people prefer stealth shipping. I've got things like a t-shirt, a pair of underwear, a pair of socks, a cup.... inside of markers, inside of other mundane items packaging. I've never got an invoice or receipt, that would help it look legitimate, but one hopes it doesn't come to that. I'm guessing 99.9% of packages reach thier destination.