Here is the system I have devised.
Participants commit to this by sending me thier address in a Private Message.
This must be done according to deadline.
On next Saturday, I will assign each person who sent me thier addy someone else's addy (anonymous addy, of course).
Naturally, if someone's uncomfortable with thier name being put on the envelope, they can substitute thier name for maybe a codeword - like a square on the bottom left corner of envelope, or something, so they know the mail is for them when they check. I'll pass along that information to whomever gets assigned thier addy, of course, so no worries.
We all mail asap.
But if you just signed up for free weed (as this is your first post) and to ruin somebody's christmas, whoever doesn't get thier weed is going to tell me, and we will all know who didn't send, so please don't ruin this for someone. So just take a second and think if 5 bucks is worth really letting someone down.
I guess it's fairest to let popular opinion decide on whether or not we let newbies in on it, but I'm willing to invest my trust a little.
EDIT: I forgot one really important problem. Someone can sign up, not mail it, and say "Oh, look, USPS must have found it/Canada Post narced on me." So I'm afraid you're out of luck, sorry.