It doesn’t apply to the same order. Coupons are off your next order. Coupon comes after transaction completion. Nemo is right.
Spend 10 to get 10 (you spend 10)
Spend 10 pay 0 (you spent zero)
Spend 10 get 10 (you spent 10)
Spend 10 pay 0 (you spent zero)
Spend 10 get 10 (you spent 10)
Spend 10 pay 0 (you spent zero)
= You spent $30 for $60 worth of gas.
So it’s basically a half off coupon however many times you manage to use it. 3 times if you bought $10 (not sure why 11) you would have gotten $60 gas and spent $30.