Basically all you have to do is signup to their mailing list from that link and then in your email you get 20%:
Thanks and congratulations.
The following is your unique code:
your code
Redeem this voucher by printing this email off and quoting your unique code when you make your next purchase.
*Not in conjunction with any other offer, Limit of one per customer per day. Valid until December 24th.
I think this means you can do it once everyday until Christmas day. Also does not look like you need to put your real info for the mailing list :P. The good thing about this is not like those 20% everything days and they take the good things away. And im not sure if ONE item or total purchase but i guess it does not matter since its basically unlimited :)
note: your code = the special generated code and given in your email