Reading the blog post it sounds like users will get a credit toward a free shirt/stickers. They will still need to pay shipping, which should be enough to keep people from getting it just for the hell of it. After a period of time, if there are unclaimed shirts they will open them up to those users beyond the first 20k.
While it sounds like they will print 20k no matter what, it doesn’t seem like they will randomly ship them out to users. They really can’t, as they don’t have the addresses of all their users.