Finished uni and no longer need to buy textbooks from the Co-op? Even when I was at uni, I found that even with the Co-op "discount", other online bookshops were cheaper.
If it's been at least 2 years since you first joined, you're entitled to have your membership cancelled and your fee refunded - this was $20 if I recall correctly, but the fee now is $25.
Forgotten your member number? No problem - see here: http://www.coop.com.au/profile/account/membercheck/
Cheers Op. And thanks Co-op for keeping my $20 safe for all these years even though I never bought anything after the first semester textbooks all of which I refunded.