20 million customers doesn't mean 20 million transactions. Considering we are talking about a convenience store I'm sure a large chunk of their customers visit every day, some probably multiple times a day.

Assuming 3.4 million customers (cash users) and 2.5 cents average loss per transaction, it would only take one visit a month for them to cross a million dollars in losses.

Of course at that scale it's not like that million or two is really making a difference to their bottom line. Doing some quick Googling their annual revenue is estimated to be $6-7 billion.