Heading says it all, it's 10C discount on Z today. Though I coudn't see it on a Z site, local Z has 10c discount for all, event not a fly buys members.
10c Discount Per Litre at Z
60
Referral Links
Z App: random (7)
Referrer gets 1 free coffee (up to 3 max).
Z Electric: random (18)
Referee gets 50L+ extra 25L. Referrer gets 50L.
The links says "Swipe your FlyBUys card for an instant 10c off".
Maybe your local Z is matching a nearby station that has 10c off, therefore if you used your FlyBuys, you would get another 10c off?