Besides doing the 20 off 50 on shoebuy.com, if you register on their site, you get another 10% off. To begin with, go through ebates or similar site to get more cash back. I just ordered 2 pairs of crocs for my kids this way and the final price was $27.80, so $13.90 per pair ~ great deal! Not to mention the cash back I got through ebates.
Since shipping is free, if you find a code for $10 off any price, then just order one pair of shoes at a time so you get $10 off each pair rather than $10 off your entire order. I think someone above mentioned that, but it's worth mentioning again.