$1 more than the other deal of $10 new user sign-up.
And relatively easier to achieve by making an accumulated min spend of $20 by 3 January 2019, instead of making an accumulated $10 cashback.
1 November 2018 - 3 January 2019 Bonus Promotion Terms and Conditions
From 1 November 2018 - 3 January 2019, you'll earn a TOTAL of $11 bonus ($5 automatic bonus + Extra $6 bonus) for every friend you refer successfully. You will receive the extra $6 bonus for every qualified referral (see above for definition of a "qualified referral") successfully referred within the promotion period. This means that the friend referred will have to make an accumulated min spend of $20 by 3 January 2019 for you to qualify for the extra bonus. Each friend you referred will also qualify for a TOTAL of $11 bonus ($5 bonus upon sign up + Extra $6 bonus after making an accumulated purchase of $20). The extra bonus will be credited into you and your friend's ShopBack account by 31st March 2019. Users qualify for this bonus promotion on an individual basis per email account on ShopBack; no accounts can be combined in order to reach the qualified friends requirement.
Ah, if only I'd seen this before signing up directly for the $10 bonus!