Along with the free credit in the email all current SCA club members have been sent was this.
Applied to your account automatically when you spend the minimum amount between 5pm and 7pm. Only for tonight, but the bonus credit expires on the 14th of October so you have a fortnight to think about how to spend it.
For ideas on what to buy, see
nocure's thread
monty.melb's thread
Maximum discount obtainable assuming a perfect spend is
$60 spend - 13%
$100 spend - 14.5%
There's also some book you can get if you spend $100
First spotted by BarneyM8
Thanks Outlander!