Just tried this for the first time and was pleasantly surprised.
It only applies to cash back rates offered by the following cash back websites; Befrugal, Capital One, Gocashback, Honey, Max Rebates, Mr Rebates, Rakuten (Formerly Ebates), Tada (Formerly Shopathome)
Swagbucks and RMN had 20% back from Chewy. TCB was offering 15%.
Checked Cashbackaholic and saw that Tada was also offering 20%.
Made the purchase through TCB and submitted for the guarantee.
TCB gave me 23% and it was quick although I still have to wait for it to clear.
Guarantee must be requested within 7 days of purchase and other T&C apply.
Today TCB has 30% back from Chewy but the $40 Chewy eGC offer expired.