As a Sharetank owner, your back-up payment card will automatically be charged if the fuel price is cheaper at the pump on the day you filled up than your weighted average Sharetank balance. Read more about savings here.
If you would like to change this setting to use Sharetank regardless if you get a savings, you can change this under the Pay by Plate / Pay in App menu.
Open Z App
Tap to open the 'Sharetank' section at the bottom of the screen
Tap on the three dots (…) in the top-right corner
Tap on 'Sharetank preferences'
Select 'Pay by plate / app preferences'
Turn on the toggle: 'Always pay with Sharetank'.
Please note: right now you need at least 15L in your Sharetank to use it with Pay by Plate / Pay in App. If you have less than 15L, Pay by Plate / Pay in App will always use your backup payment method.