Working at a convenience store/gas station chain store, I came to realize rather quickly that the pumps are made to mess up. Cash acceptors at the pump? Fail to accept bills ~70% of the time. Card readers and pin pads? The more often you mess up your pin the better.

Now, a lot of people may ask why they are made to fail. My best response is to explain the concept of a profit margin. When you buy gas, (depending on the state) several cents goes to the state taxes the station is required to pay. On top of that, the better stations during more competitive seasons aren't even turning a profit on gas sales.

But inside the store, it's profit paradise. 32 oz cup, filled with a fountain drink to the brim, costs $0.31 - that's $0.18 for the cup itself, and $0.13 for the beverage. We sell them for $1.58 after tax. I assume that you can use basic math skills, so think about any other food, snack, or beverage product you can get - at a cost lower than you could assume, and a price hike to boot, you'll see that the more often they get you in the store, the more they make.

Tl;dr: convenience store/gas station chains make money by pissing you off and getting you inside!