Thanks to Lynx-lynx and ChandaK562 I managed to get a couple of 90% off codes

, and I can confirm that only one 90% code will work per account.
They also definitely seem to note whether this was your first purchase ever, and I would guess they use this to determine whether to send a 90% code back to the referrer?
After one of my purchases, there was actually a banner for the refer a friend promo (on the U.S. site), but this link also didn't work from South Africa - it was similar to those previously posted.