Activate in the app, Earn More tab > Limited time challenges or on the ShopBack Plus page.
Excludes purchases at Amazon Australia, Survey Wall, ShopBack Surveys, Savvy Surveys, Compare Club, Exetel (Compare), Optus Broadband (Compare), Tangerine Broadband (Compare), OVO Energy (Compare), Compare Electricity and Gas, Compare Broadband, Bupa via Compare Club, nib via Compare Club, ahm via Compare Club, HCF via Compare Club, Frank via Compare Club, Australian Unity via Compare Club, Peoplecare via Compare Club and ShopBack Giveaways.
Last time SB Giveaway purchases did complete the challenge (even though the T&Cs say otherwise), I have bought $20 again and will report back if it's still working.
Edit: It worked, tracked within minutes and the challenge completed in ~6 minutes. Only got $1 this time :(