Valid on the first order and once per customer, can be used on sale items. Says "limited time only" but there is no end date and it is listed under the "ongoing" section on the T&C page.
Equivalent to 20% off $100/250 or 21.43% off $350
Seems that if a new member was to sign up for the mailing list 20off99 code, they could potentially use one of these codes on the first order and then the 20off99 code on the second order. Haven't tested, if true will update.