I went to the Co-op bookshops today at Melbourne University, and they had signs up everywhere with 20% off store wide for them hitting over 2 million members (or something like that, I can't remember).
Anyway, 20% off applies off RRP for the full priced items (not off the 10% offer for members) and sale items, and only available for members.
Edit: New info posted in link.
The photo isn't bad, they gave you a pretty lousy receipt.