Don’t worry about using less than the amount you you told the cashier, either. You will initially see the full amount charged in your app, however within a day or two, it will adjust to the correct amount. You can check your transaction history to make sure you were only charged for the gas that you pumped.
That's it! With this small trick you should never run into troubles, at least when it comes to filling gas.
If you run into any other issues like a flat tire, we may not be able to help you but you can still send us an email at firstname.lastname@example.org anytime.