I think Operaguy is right.  The $30 voucher is for those who buy the annual membership (or renew annually).  Sorry, I've been in the PC since before there was a monthly option, and tend to forget about it.

I usually renew in November, on the PC anniversary, but I held off this year, so the voucher would still be good when V4 came out.  (Due to the various free months DAZ has given me over the years, my membership doesn't actually expire until March.)  I just renewed annually, and a $30 voucher was credited to my account.