I actually have no idea. I didn't sign up for anything.
The day after I turned 18 last year, I got a Mach3 in the mailbox in a package that said "Congratulations from Gillette on turning 18" (or something to that effect). It was nice, because it's not some cheapo plastic disposable, and actually works really well.
Then a week or so later, another one came in the mail in the same package.
Apparently, some of my friends received these on their 18th birthdays too, except they got one each.