I decided to try Kuapay app after I got a flyer from the local Shell gas station in Marina del Rey (Marina Shell Service Station). The flyer said I can save 50 cents per gallon at that gas station by using Kuapay app. I went home and researched. Kuapay has apps for iPhone and Android that let you pay using your credit card but without pulling the card out.
I installed the app on my iPhone, entered one of my credit cards into it and was good to go. The app makes you enter a pin to secure itself. In other words, on every app startup, you have to enter the pin to the app, otherwise the app won't start. This is a security measure.
Since I no longer have the service for the iPhone, I actually brought my Nokia Lumia 521 T-Mobile Windows Phone and used it as a mobile WiFi hotspot to use the iPhone and the Kuapay app. I would not have to jump through hoops to make the Kuapay app work if there was a version available for Windows Phone (WP8), but currently there isn't one.
If you have an iPhone or an Android device, you will not need to do anything special.
Once I got to the station, I scanned the QR code on the pump with the scanner within the app (using the phone's camera) and was asked how much money do I want to spend. I chose to spend $41, which is enough to get 10 gallons. The app deducted about 12.5% from that amount. So I got $41 worth of gas for $35.88. That is savings of $5.12 for 10 gallons.
In addition, if you join the Fuel Rewards network, you can get $.03 off a gallon at some Shell stations. And most rewards credit cards give you at least 1% cash back. As a result, the next time I used Kuapay (with gas at $4.10 per gallon), I saved .51 + .03 + .04 = $0.58 per gallon.
Update 07/2014: if you specify the amount that is too high and don't pump enough gas to use up that amount, the change will be credited back to your credit card automatically. I tried and it works - a couple of days later you get the refund of the difference.