I decided to look at the 7-Eleven app as it keeps getting recommended. There does not seem to be any option to actual create an account. The closest I can find is to sign up for their rewards program, which starts with requiring a phone number and tells me that my phone number is invalid. Also looking at the fine print they now limit the amount you can save by locking the fuel price to 25c, which considering it often jumps 40c or more when it goes up these days is not quite the "price lock" I imagined.
Edit: solved the issue with signing up - some weird glitch on the tablet that if the app is in landscape mode it doesn't show the option to register. It is there on the phone and when I resize the window on the tablet it magically appeared there too!
2nd Edit: solved except it is supposed to send a code to my email address and it has not come through.